I would ask whether you realize you're on a linux community, but you referred to a man page as a wiki article so you are clearly lost.
The first paragraph past the link is a summary of the function of the program.
fstrim is used on a mounted filesystem to discard (or "trim") blocks which are not in use by the filesystem. This is useful for solid-state drives (SSDs) and thinly-provisioned storage.
muntedcrocodile
Killercat103