An explicit row lock.
Row locks can be explicitly acquired in order to serialize edits to a given row. This feature may disappear from HBase in the future, so try not to use it if you can.
While a row is locked, no one else can edit that row. Other concurrent attempts to lock that row will block until the lock is released. Beware that the blocking happens inside the RegionServer, so it will tie up a thread of the RegionServer. If you have many clients contending for the same row lock, you can literally starve a RegionServer by blocking all its IPC threads.
Row locks can't be held indefinitely. If you don't release a row lock after a timeout configured on the server side, the lock will be released automatically by the server and any further attempts to use it will yield an {@link UnknownRowLockException}.