Basically, it's this. Stuff falls down a black hole, but the properties of a black hole mean you can't tell what stuff fell in(no hair theorem). Information, as it's called. Stephen Hawking discovered that black holes emit tiny amounts of radiation due to quantum effects, and this is called Hawking Radiation. Over time it "steals" energy from the black hole, and it gets smaller and smaller as a result. Eventually it will release the rest of its mass as a powerful but brief blast of energy. Problem is... we still have no idea what fell in... and that's the Black Hole Information Paradox, and it has huge implications for causality. If you don't know the information about what went in... well... ANYTHING could have happened. And that causes causality to fall apart.
I've seen it, it's quite good, although his books are much better. Read his books. A Briefer History in Time is a real good one, also, The Universe in a Nutshell.