Check consistency of a btrfs filesystem, and optionally repair some types of breakage. Can only be run on an unmounted FS.
Note that while this tool should be able to repair broken filesystems, it is still relatively new code, and has not seen widespread testing on a large range of real-life breakage. It is possible that it may cause additional damage in the process of repair.
If you have a broken filesystem, you should probably look at the recovery or repair tools.