Five Things… is a fanfiction writing form or structure that originated in Smallville fandom in 2001 with the fic Five Things That Aren’t True by Basingstoke. The format requires a set of multiple scenes that are related to each other in some way, without necessarily being in the same chronological timeline. The number of ‘Things’ may vary slightly depending on the work (see Evolution & Variations), but five is the most common number. 5+1 Things is an enduringly popular variant.
An early example is Five Easy Pieces and a More Difficult One, a Blake’s 7 story written in 1999 by Nova. It is online here.