Operational Lock, or Op-Lock, is what SMBServer Message Block is the file protocol developed by Microsoft. The now obsolete and deprecated version 1.0 was known as CIFS. Primary file protocol for Windows and OS X. More Clients use to indicate byte ranges in a file they are caching without actually preventing other users from writing.
When another user overwrites data, a client has op-locked the server sends a notification invalidating the cached data.