fstrim is used on a mounted filesystem to discard (trim) blocks which are not in use by the filesystem. This is useful for solid-state drives (SSDs) and thinly-provisioned storage.
But why it is used in this Android App?
Almost all Android devices use NAND chips for data storage. Because of how NAND chips work, they may become significantly slow after some time. Trimming would help to restore NAND Chips performance to their initial value.