We don’t know exactly, there are lots of theories. Because no light, or anything else, can escape from a black hole, there’s no way to know what’s going on inside. So all we can do is look at how things behave very close to the black hole and try to use this information to work out what it might be like inside. This is pretty hard though, which is why there are so many different theories!
We know that gravity slows time down, which means as you fall into a black hole time stretches out more and more the closer you get. It’s possible that it stretches out infinitely, meaning you never actually get inside the black hole at all, and all the stuff that falls in is actually stuck at the surface.
Inside the black hole, the laws of physics we know break down though, so there are a lot of things that could happen. One popular theory is that the dimensions of time and space swap, which means that while normally you can move in any direction in space, but only forwards in time, inside the black hole you would only be able to move in one direction in space (towards the middle of the black hole), but could move forwards or backwards in time. This is why some people think black holes could be used as time machines. I’m not sure though personally, as there’s no way to get out again! The answer instead is probably worm holes, which can make time machines and which you can get out of 🙂