This is a sophisticated question. The proper place to start in answering this question is with a brief examination of structuralism. Structuralism is a movement that roughly started in the middle of the twentieth century in Europe. It is a basic belief that societies have a deep structure that is roughly modeled on language. Often times within structuralism is the idea that structures are based on binaries. To put it another way, structures in society are based on opposites.
Post-structuralism, then, obviously disagrees with the idea that society or anything else for that matter is based on binary opposites. They posit that this is too simplistic a view and very superficial. Society, they argue is much more complex and the human mind is much more complex as well. Derrida, Foucault, and Butler are probably the three best known post-structuralist thinkers.