Re-checks every track's recorded source (YouTube/YouTube Music/SoundCloud link) against its own library tags — catches a title collision with a different artist's song, or a Let's Play/compilation that slipped in under a correct-looking name, even for tracks downloaded before those checks existed.
Scan
Each check is a real network look-up, so a full library takes a while and runs in the background — this page checks in on it periodically. Click again any time to continue checking whatever's left.