Yeah, the man page doesn't really help me out.

   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.

   By default, fstrim will discard all unused blocks in the
   filesystem. Options may be used to modify this behavior based on
   range or size, as explained below.

   The mountpoint argument is the pathname of the directory where
   the filesystem is mounted and is required when -A, -a, --fstab,
   or --all are unspecified.

   Running fstrim frequently, or even using mount -o discard, might
   negatively affect the lifetime of poor-quality SSD devices. For
   most desktop and server systems a sufficient trimming frequency
   is once a week. Note that not all devices support a queued trim,
   so each trim command incurs a performance penalty on whatever
   else might be trying to use the disk at the time.

For instance, why would unused blocks not be discarded? And what does "discarded" even mean in this context? But it does recommend against using it for SSDs so I think I'll skip it.