What is postmodernism? What is post-structuralism?

Postmodernism is, by its very nature, difficult to define since it basically amounts to a rejection of traditional categories of knowledge. Basically, postmodernists call any universal truth, narrative, or point of consensus into question. They argue that our experiences and understandings are subjective because each person comes to these understandings from their own perspective. In history, for example, postmodernists would argue first that modern people cannot really understand the motives and perspectives of historical actors, calling all historical interpretation into question. Second, they claim that there can be no grand narrative of history because there are too many perspectives involved in any historical moment. Basically, postmodernists, whose work has informed almost every academic discipline, question the very assumptions disciplines are based on. 
Post-structuralism also tends to focus on the individual and denies the authority of academic disciplines—especially texts. One of the major ideas associated with post-structuralism is the notion that texts (nonfiction or fiction) cannot have a fixed meaning independent of the reader's interpretation of them. Accordingly, the reader is as important in assigning meaning to texts as the author. In his famous essay "The Death of the Author," Roland Barthes, one of several literary critics closely associated with post-structuralism, claimed provocatively that the "source, [the] voice" of a text "is not to be located. . . this is because the true locus of writing is reading." As a result, he claimed that post-structuralism heralded the "death" of the author because the reader created meaning as much as the author.
Proponents of post-structuralism in particular are notorious for being difficult to read for anyone other than an academic specialist. Despite this, their work, along with that of the postmodernists, has been influential in literary criticism, history, and many other fields. 
https://plato.stanford.edu/entries/postmodernism/

https://www.philosophybasics.com/movements_poststructuralism.html

Post-modernism and post-structuralism are 20th century artistic, literary and philosophical movements that can be understood by differentiating them from the movements they succeeded. 
Modernism reflected the ideas that there are universal truths, and that progress and technology are inherently good. The movement also embodied the notion that art and popular culture were distinct. Post-modernism reflected a more skeptical sense that meanings could be ironic and that art could be taken from mass culture.
Structuralism was based on the idea that human actions can be understood by recognizing there is an overriding "structure", apart from what we see, with which all human activity connects. Post-structuralism rejects the notion of "structure" for the idea that culture and society are great influencers of human actions.