Post-structuralism is an important movement, which seeks to correct the assumptions of structuralism. Structuralism believed that the world and knowledge was based on a system of thought or a structure. More specifically, they posited that the world was based off of binaries. So, if a person studied the system long enough, he or she could understand the features of the structure.
Post-structuralism went against this and posited that these structures did not exists and these structures were superficial. Derrida and Foucault are two important authors who are considered in the post-structuralist camp. For example, Foucault published a great article speaking of the archaeology of knowledge. His basic point was that there are no systems and that all sort of knowledge are legitimate. His point was that there was an insurrection of subjugated knowledge.
The historical importance is that the world is no longer seen from the point of view a of meta-narrative, which is shaped and controlled by those in power. It questions the absolute claims of the institutions and democratizes power and knowledge.