The Wikipedia article on Postmodernism has an excellent definition:
Postmodernism is a movement away from the viewpoint of modernism. More specifically it is a tendency in contemporary culture characterized by the problematization of objective truth and inherent suspicion towards global cultural narrative or meta-narrative. It involves the belief that many, if not all, apparent realities are only social constructs, as they are subject to change inherent to time and place.
Recasting those sentences as three simple statements creates an even better definition: 1) Objective truth is problematic. 2) Many apparent realities are only social constructs. 3) Global cultural narratives (Absolutes) should be treated with the utmost suspicion.
(Another way to say global cultural narrative: "A narrative that remains valid in every local culture." So far I have yet to see one of those.)
All three of these statements have two qualities: (1) they are open to verification and (2) it's useful to know whether they are true and false.
Those two qualities, in turn, are interesting. They just now came to me, no doubt bubbling up from things I've read long before. But they are a pretty handy tool for selecting among the many questions clamoring to be answered.