Post-modernism can be seen as a stylistic movement in the arts, often specifically associated with literature. Post-structuralism can be seen as a loosely parallel movement in critical theory of the mid-20th century into the late 20th century emphasizing the notions that meaning is contingent on context, culture and nuance and that language is an ideological vehicle.
Meaning, like language, becomes unstable in such a conception. This idea informs the stylistic devices that are characteristic of post-modernist literature. We can see from the terminology here that both post-modernism and post-structuralism are related to the concept of dissolution.
In modernism, the idea of subjective reality oriented much of the work of the era, expressed stylistically, for example, in the use of stream-of-consciousness narration. This concept looked at individuals as products of personal history and rendered individual psychology as a unique existence/dynamic.
Post-modernism took this concept a full step further, proposing that even the individual's consciousness (opinions, attitudes, beliefs, ideology, etc.) is constituted by competing forces derived from language. The individual thinks through language, conceives of the world with words, etc.
Even these words are not stable in the sense that they cannot convey an unalterable message. The consequence of this is that once language is destabilized the resultant knowledge that comes from that language is no longer a stable product. The end result therefore is that there can be no universal truths upon which to base an understanding or a social construct.
The nature of langauge being what post-structuralists suggest it is (not unified by a single structural organizational principal and not dominated by any one over-arching dynamic or value system) then the "truths" derived from such a language will itself be something other than unified, organized, or singular. Truth will instead become relative, contextually defined and subject to challenge.
This dissolution of the truth is often communicated as the movement from capital "T" truth to lower-case "t" truths (plural).