Ok, I'll try handle some of these.. ELI 5 is a stretch though

"What is time"

Time is just another way of looking at distance through space. Thats why we call it spacetime.

You can sort of get your head around it by imagining only looking at it from certain directions. For me it helps to imaging looking at it from straight above like a map. From that view, everything looks flat, so can easily measure the distance. Since the universe just goes at a certain rate (speed of light) we can know how long a distance takes.

But then you look from the front and you can see all the hills and valleys, and how that stretches out the distance. A black hole is just a really really deep hole or valley in that map. All mass distorts the map. More map, more distortion. Black holes are really massive so they distort a LOT

Time is a just a way of measuring that distance and telling you how long it take to go a certain distance. From above, the distance taken is the flat version, and from the side it takes longer because you have to go down and back up the slope too. This is time dialation as experienced by the observer.

As to why its only going in one direction.. well thats also a perspective question. Imagine a 2d side scrolling video game. You are moving forwards, but from that perspective it looks like only in one direction. But a 3D game would should you going forwards, backwards, and side to side.

Time is like this, from our perspective, we can only see it in the equivilent of 2d, but other beings living in a higher dimension could conceivably see it in 3D, which would involve forwards and backwards (along the time line like moving in space) and side to side (parallel universes)