PWP stands for Plot? What Plot?
PWP fics have no plot at all.
Typically a fic labelled PWP will be an explicit scene, written as if taken from a larger work or considered to take place in the same continuity as another fic, thus removing the need to actually establish an in-fic plot. Commentfic, flashfic, and other short forms are often PWP.
Kinkmemes are filled almost exclusively with PWP, both good and bad.