MST, a term shortend from the title of the televison series their concept is based on (Mystery Science Theatre 3000), are generally stories in which various characters (sometimes those of the television series) mock a work of published fan fiction. This usually takes the form of the mocked story being reposted with comments from the characters inserted as the story progresses.
“ In MSTing, the author picks a badly written piece of text — usually a Usenet post, web page, or some other source such as a rant, spam or fan fiction — and inserts mocking comments from fictional readers of the text, essentially writing a script as if the MSTing were a movie. While "standard" MSTings attribute these comments to the three main characters of the MST3K cast, others might use characters - usually (though not always) from the universe of the story being mocked. Often a prologue, epilogue, and intermissions are added in which the characters discuss a topic on the same theme as the original text, although intermission segments are usually dropped if the original work is short. ”