Fine.
It's defined as having doubts about whether or not humans can conclusively know things about the world. This isn't really a new idea, but what we know as post-modernism today is skepticism about modernism. Modernism, of course, is the notion from people like Voltaire and Rousseau which thinks we can form universal theories about the world through observation and reasoning, as well as that these things progress in a linear fashion, especially history. This works well for things like medicine and plants, but note that Hume originally pointed out a skepticism of modernism by arguing that there is only so much we can know about causality, morality, and the self. This is because from a strictly empirical perspective these aren't things that you can actually observe. Then Weber, Durkheim, Comte, Spencer etc. tried to find universal scientific truths about humans.
So while modernist attempt to create universal theories about history, humanity, reality, etc., post-modernism suggests this really isn't possible. So you have people like Foucault who wrote that the intellectual history of things like psychiatry, history, etc. shouldn't be understood as progressing in a linear fashion towards freedom and scientific truth, but instead as shifts in power shapes populations and liberal institutions. Or Rorty whose concept of liberal ironism stated we should continually radically doubt our own understanding of the world. For example, the notion that hierarchies are "natural" instead of something shaped by populations and institutions is an example of a post-modern belief.
But I'm not a post-modernist, your double digit IQ friends don't know any of this stuff and that's why I make fun of them, and I'm not doing any more of your Soc 335 homework for your online course.