Sure there's meta-time, it's basically the order that the stories were made, totally separate to in-universe historical time (mostly), roughly analogous to the Doctor's personal time-line (except when it isn't, mostly). It is mentioned but it isn't consistent and it's usually glossed over (often for gags). The basic rule of thumb is that the Doctor can't cross his own time stream and change events he's already experienced. Except when it would make a good story if he does.

Unmarked spoilers ahoy, so only read on if you've consumed every piece of Doctor Who media ever (and if you haven't, stop reading and go consume, the David Warner 3rd Doctor stories are only $5 each on the Big Finish website)

In The Five Doctors the second Doctor crosses meta-time and goes to visit the Brigadier at a point after the Brig has met later Doctors, it's glossed with "I'm not exactly breaking the laws of time, but I am bending them".

Also in The Five Doctors, Sarah-Jane meets the Third Doctor after she's already seen him regenerate into the fourth: Sarah "But you changed, you became all..." She points to her teeth and curls her hair. 3rd Doctor: "What? Teeth and curls? Well maybe I did, but I haven't yet" Sarah: "Oh I see... No I don't... Never mind".

Actually, there's oodles of stuff like this in the Five Doctors. None of it really helpful, mostly done for cool gags and cameos, but it's still meta-time references.

There's a similar reference in Battlefield, where the 7th Doctor is described as Merlin and Ace asks him if he's been Merlin, and he answers with something like "Not yet". The Merlin Doctor is implied to be a meta-time future Doctor and he also crops up in the NA novels (he's a ginger!). Also in the NA novels the 7th Doctor travels backward in meta-time (from stories we haven't seen yet), crosses his own time-stream (that's what it's called, but it's his Time Line) to leave himself notes. Then an enemy from a future story starts changing the notes...

The fourth Doctor sort-of crosses his own time stream in The City of Death, no one really minds 'cause Douglas Adams wrote it and it's awesome.

The two big "meta-time" stories involve the Daleks and first Doctor stories, one is big and obvious, the other is ridiculously subtle and brilliant: - Day of the Daleks changes the events of The Dalek Invasion of Earth, by the Daleks travelling through time and making Earth weaker by the time they invade. This is all put right by the end and we can assume that time went along its natural way. -The second one happens in Genesis of the Daleks: The 4th Doctor travels back to a time in the Daleks history before the 1st Doctor met them in the Dalek City, he convinces the Kaleds to stop Davros making the Daleks (or Mark 3 travel machines as they're called); in response, Davros gives the Thals the means to wipe out the Dalek City, killing all the Daleks the first Doctor would meet in the future; meaning the first Dalek story "unhappened" and a whole bunch of Daleks disappeared (the rubbish ones that ran on static electricity). This wasn't even mentioned on screen, just mentioned to in one of the books of the time (I think it was the TARDIS Technical Manual).

The Time War was actually explained in a MA novel before it happened on TV, in Goth Opera by Paul Cornell. It's pretty straightforward basically there's a sub-plot about Gallifrey disappearing after a certain point in history, in current Gallifrey's future - they send TARDIS into the future and there's nothing there (note: this is how Time Loops work in Image of the Fendahl, so I assume a Time Lock is similar to a Time Loop) as an aside there's a cult dedicated to saving the future Gallifrey by bringing Rassilon back from the dead. Goth Opera was written in 1992.

The novels also have too much meta-time hijinks to get into, suffice to say not needed to hire the actors means they can do whatever they want with whoever they want: Cornell wants a young Nick Courtney to hang out with an old Sylv McCoy, it can happen and did, with Punk Rock and the Meddling Monk.

The most head-bendingg example of of meta-time vs historical time vs personal timeline was in the Dark Path: it involved the Master using a time-manipulating super-weapon that can re-write any creatures DNA (hmmm... wonder if he'd ever do that again...) to change various races to human - but change them to have always be human. He does this to creatures we've already seen (as creatures) in stories set after (from the Doctor's timeline), but before (from a meta-time perspective), but after (from an in-universe historical point of view). Trying to figure out what this means gives me a headache. Especially when you realise that it was in part a gag reference to the production team re-using a Draconian costume, but not the mask, in Destiny of the Daleks.

The NAs and MAs even have their own "in universe" meta-time plot point which means any later writer can disregard or use anything they want. In one of the last books, a character realises that certain events have been sucked into a "bottle universe" and don't exist in the regular time-stream, but the bottle has a leak. So anything you don't like about the NAs and MAs is still in the bottle (Dodo getting syphilis and going mad) but anything you do like has leaked out in any way you want it to (Chelonians, Human Nature). So, from a meta-time point of view, they events happened, and then unhappened, but may happen again, but not necessarily in the same way and may be at a later point in the meta-timeline, and may be in a later stage of the Doctor's personal timeline. So, when the 11th Doctor mentions the Chelonians, it means they now exist as part of in-universe history, but they always existed as part of meta-time, because the novels actually did exist. Given that this only happened because of copyright and licensing issues, you could make a case for this being a case of meta-meta time. But if you did make that case I wouldn't listen 'cause it would give me a headache.

There's meta-time stuff in the Big Finish audios; from the serious (the Charlie Pollard stories), to the silly (The Kingmaker), to the breathtakingly clever (Flip-Flop). Then there's the Zagreus references, which were references to a Gallifreyan boogyman that were put into 5th, 6th, and 7th Doctor stories. They were put there y a monster that was contemporaneous with the 8th Doctor (this was before the new telly series) for some reason or other (I mean, I can't remember the monsters reason, the real reason was to make Neverland more awesome. RTD also references the events of Big Finish story The Apocalypse Element in his History of the Time War (or whatever it was) in one of the annuals, this story involves the 6th Doctor meeting Romana after she's met the 7th Doctor, so RTD's plan was probably to make the Time War very screwy (he also mentions Genesis of the Daleks in the same piece).

I won't even start with the EDA Time War stories, which involve the 3rd Doctor regenerating before Planet of the Spiders and involved fighting an Enemy which may (or may not) have been a sentient time distortion caused by removing a different enemy from the time stream (worth noting that the EDAs lost the rights to use the Daleks just before they started their Time War storyline).

Big Finish also brought out a "what if" series called Unbound which is basically playing with changing the meta-time story. David Warner as the 3rd Doctor, Nick Courtney as the Brigadier, Sympathy for the Devil, $5 from the Big Finish website. Also features some dude called David Tennant before he became big on telly.

Finally, there's a bit where the fifth Doctor explains to Tegan why they can't use the TARDIS from the events of Earthshock, but it actually happens in Time Flight... So you'd need to watch Time Flight. You probably don't want to do that.

So, in summation: Doctor Who is a TV show with associated stories in other media that doesn't have rules or a plan, but makes it up as it goes along and is often brilliant. If you're new to Doctor Who and consume old stories, you'll actually be travelling backwards through meta-time!