Scrub is a pass over all filesystem data and metadata and verifying the checksums. If a valid copy is available (replicated block group profiles) then the damaged one is repaired. All copies of the replicated profiles are validated.
Scrub is not a filesystem checker (fsck) and does not verify nor repair structural damage in the filesystem. It really only checks checksums of data and tree blocks, it doesn’t ensure the content of tree blocks is valid and consistent. There’s some validation performed when metadata blocks are read from disk but it’s not extensive and cannot substitute full btrfs check run.
The user is supposed to run it manually or via a periodic system service. The recommended period is a month but could be less. The estimated device bandwidth utilization is about 80% on an idle filesystem. The IO priority class is by default idle so background scrub should not significantly interfere with normal filesystem operation. The IO scheduler set for the device(s) might not support the priority classes though.
The scrubbing status is recorded in /var/lib/btrfs/ in textual files named scrub.status.UUID for a filesystem identified by the given UUID. (Progress state is communicated through a named pipe in file scrub.progress.UUID in the same directory.) The status file is updated every 5 seconds. A resumed scrub will continue from the last saved position.
Scrub can be started only on a mounted filesystem, though it’s possible to scrub only a selected device. See btrfs scrub start for more.