Black holes are sites of immense gravitational attraction into which surrounding matter is drawn by gravitational forces. It was originally thought that the gravitation was so powerful that nothing, not even radiation, could escape from the black hole, but Hawking theorized that (particle-antiparticle) radiation would be emitted from just beyond the event horizon. This radiation does not come directly from the black hole itself, but rather is a result of virtual particles being "boosted" by the black hole's gravitation into becoming real particles. This would sap some of the black hole's energy, and so when these particles escape the black hole would lose a small amount of mass.
See also: primordial black hole.