Postmodernism is a movement, or style, or practice, in art and literature, which came after modernism. Whoever coined the term "modernism" was evidently not a "planner ahead", and hence all future cultural movements are condemned to have increasingly silly names. Perhaps they'll start from scratch some day.
But what is it? Well, as my professors used to say, "That's a very good question. A very good question indeed. Very very good. Top question. Oh, is that the time? I think I'm supposed to be giving a lecture or something. Look, bunnies!"
The idea, or one of the ideas, of postmodernism, is that we live in a world saturated by fiction, and in our day-to-day lives we experience as much, if not more, fiction than real life. Indeed, as the consumption of fiction becomes the principle human activity in the West, it is arguably becoming more "real" than other forms of behaviour. Thus, since real life has been done to death in books, movies, and TV shows, it is perfectly legitimate to write books, movies, and TV shows about other books, movies and TV shows.
Or, in other words, if you can't come up with an original idea it's OK to copy someone else's.