The laws of general relativity allow for the possibility that there could exist closed timelike curves, in which spacetime curves around so that an object ends at the same spacetime coordinates that it was at earlier. In other words, it allows for time travel (in some very rare, highly contrived situations, which are probably impossible to duplicate).
Technically, Hawking's chronological protection conjecture allows for minor time travel at the sub-microscopic levels, where it has no real effect on anything substantive.
Hawking's 1992 paper presented the idea of a chronological protection conjecture through a sort of metaphor with a science fiction-like "Chronology Protection Agency." Other papers (such as this 2002 paper by Matt Visser) have since addressed this concept, trying to elaborate further aspects of why the laws of physics would require a chronological protection conjecture.
