Modernism is Descartes` "I will find a single indisputable axiom and can prove everything knowable from there." His axiom? "Cogito ergo sum."
Postmodernism is rooted in the notion that there is no indisputable base axiom from which we can build our understanding of the world. It's unprovable assumptions all the way down. The consequences for this, if you accept it (you don't have to, but you're going to have a hell of a time proving you're right), are ENORMOUS and affect virtually every field of human understanding.
In the sciences we tend to start from the assumption that what we measure is real. As a practical matter, that's fine, but there is no sound rational basis for that assumption. It's an unprovable assumption.
All of our value systems (and by extension, our social systems) are founded on unprovable assumptions.
This is the essence of postmodernism, in so many words. There are no sound assumptions. The best we can do is say what follows from one assumption or another, but we can never say which conclusion is right because we can never say which assumption is right.
The implications of this can be particularly disturbing for rule-oriented individuals, but that doesn't make it wrong. It's irrational to claim that some assumptions are better than others without some basis (in which case, they are no longer assumptions).