In short, post-structuralism refuses the idea of objective knowledge or indeed, that anyone is able to know him or herself.
Post-structuralism "deconstructs" (to use a term coined by Jacques Derrida) the old structuralist notions that have always dominated in the western world: mind vs. body, literal vs. figurative, presence vs. absence etc. In fact, post-structuralist critics claim that these simple binary codes are not only unnatural, but that they are indeed constructions contrived by man and the discourse he uses. In post-stucturalism, "A" and "not-A" can co-exist.
Thus, with a post-structuralist mindset, a reader should challenge foundational assumptions and ideas that are ordinarily accept as being "true" or "right". He should question his notions of what "objective knowledge" is and and not rely on the idea of there being an "absolute truth".
In fact, every text is tainted by "discourse", that is, language which has been influenced by particular historical, political, social, cultural etc. conditions. Thus, since the language that was used to write a text AND that is used to read a text is tainted, it becomes impossible to find "meaning" or "truth" in a text. The text is "destabilized" because of discourse.
As a consequence, any piece of literature can never really be "interpreted".