what are black holes and what do they have to do with time travel?

A black hole is defined to be a region of space-time where escape to the outside universe is impossible. The boundary of this region is a surface called the event horizon. This surface is not physical, just an imaginary boundary. Nothing can move from inside the event horizon to the outside, even briefly. The existence of black holes in the universe is well supported by astronomical observation, particularly from studying X-ray emission from X-ray binaries and active galactic nuclei. It has also been hypothesized that black holes radiate energy due to quantum mechanical effects known as Hawking radiation. more info here: http://en.wikipedia.org/wiki/Black_hole