It says scientific facts aren't nearly as knowable as we think they are. There's some validity to this approach. However, by disputing scientific facts, some postmodernists have become downright silly. They contradict the obvious reality around them. Blacks, teens, gays, women, Jews, etc., may see a different reality than us old white males - but if a black lesbian steps off the roof of a building, the law of gravity will apply, regardless.
Hard science is real.
The philosophical postmodernists, a rebel crew milling beneath the black flag of anarchy, challenge the very foundations of science and traditional philosophy. Reality, they propose, is a state constructed by the mind, not perceived by it. In the most extravagant version of this constructivism, there is no 'real' reality, no objective truths external to mental activity. The perceptions may be false, but there's no doubt that my conscious mind is receiving them. It's similar to Plato's example of a person who can see only shadows in a cave, but can't see the objects casting the shadows, and thus can't be sure what they are.