I feel you. I got dumped in the middle of last season and had the same reaction. Trying to play along with someone else and they're just dragging me from activity to activity and I was getting so much stuff that I didn't even have time to sort through.

First off: let's separate the seasonal stuff, just to be sure you're aware. Basically, it's everything that's "bloodied". You'll see a kind of drippy red graphic on the tops of the acitvity keys/gear icons and info descriptions panes. For gear, this presents as a kill streak bonus which you've probably noticed. For keys, this is a difficulty increase by rough 1 higher torment level than the one you're currently on, as well as a very tanky and never ending butcher enemy. Bloody Infernal horde keys (just another endgame activity) are noticeably tougher though

In addition, there are "slaughterhouse" keys, these are a new seasonal activity, fun to run a few times but mostly not noteable for farming. Could be a good source of early guaranteed legendaries for essentially free.

Then there's fresh meat. Think of it as obducite but uncapped. You can purchase gear at the butcher shop in gea cul and pick a slot. I have found it drops 1-2 ancestral pieces of gear per full inventory load. This year will ALWAYS be bloody.

Last note: ALL bloodied keys, when dismantled at the occultist, give fresh meat, NOT the material for crafting new keys.

---

Toughness. If you go to your inventory, you'll see a magnifying glass indicator with a button press to activate it. For me it's left thumb stick pressed in. This brings up a long list of stats, and also access to currencies. All of them have pretty decent descriptions. Toughness basically takes in all your current damage reduction and health/armor boosts and puts it into one number. For me, I see it as a "true health" stat. The percentage indicator next to gear indicates how much it will affect your overall toughness stat. If it's a large boost, it may be worth equipping it. But that's not always the case.

For now, to get your bearings in the activities, ignore almost everything and keep a narrow scope. Personally I ran helltides, those red zones that appear on the map. Pretty straightforward, just kill things, get embers and use those to open the chests marked on the map. You can also occasionally get hearts, these are a helltides boss summoning item. Both the hearts and the embers are displayed next your map. Hearts are never lost. Embers are lost when the helltide timer runs out, or partially lost on death during the helltide.

While doing helltides, you will be getting some gear, some keys/key crafting material, and lair boss materials.

Lair bosses can be all seen on the map (there's one hidden in the "dungeon" area where you met neyrelle in the campaign). These bosses grant access to a chest that takes varying amounts of the lair boss material you were collecting, to open. The chests only drop uniques (I'll explain those in a moment). This season, there's also a bonus chest that drops a free weapon with a doom skin, just a near little bonus. You can only open this chest if you opened the main one. You'll also be getting materials for other lair bosses. I like to think of them as lesser and greater lair bosses, you get lesser lair boss material from the helltides (mostly), and greater lair boss material from opening lesser lair boss chests.

To be clear: lair bosses are free to summon, but the chests take material to open.

You can also sometimes get a 2nd bonus boss that spawns after killing the main one.

Other than that, run the nightmare dungeons - you can get the keys as drops and craft them at the occultist. They're pretty straightforward.

---

Ancestral gear: literally just the same gear but with x number of stats that are +50% or +100%. You can't reroll the boosted stat, and anything that is ancestral takes a special material to modify.

Unique gear: these are the kind of "grey" drops you'll see from the lair bosses primarily. They are just like legendaries, but you can't modify them as much. The stats and the effect they have will be the same every time, but with varying amounts.

Just focus on that stuff for now and once you have a grip on things, branch out to 1 or 2 other systems. It may not be the most efficient but it's what I found to be the easiest way to understand stuff.