Hawking radiation is thermal radiation emitted by black holes. It is named after Stephen Hawking, who theorized its existence in 1974, although credit is often also given to an Israeli physicist named Jakob Bekenstein.
A simplified explanation of Hawking radiation is the simultaneous creation of a particle-antiparticle pair at the event horizon. Under normal conditions, this particle dual will, after spontaneous creation, follow a path that leads to their mutual destruction. At the event horizon, however, one particle falls into the black hole, while the other escapes outside of the event horizon. Following the law of conservation of energy, the particle inside the event horizon must contain negative energy (from an observer outside of the event horizon). Thus, the particle that escape is emitted as radiation.