File with a .LOCK extension contains information on blockage of another file, a database or a resource, for instance a device. It is similar to an LCK file. Parameters of a blockage may be stored within it, however, it may be empty and function as an indicator for a blocked file.

LOCK files are used in case of applications which have to provide an exclusive access to specific files, databases or devices. For instance, they are utilised by the Apache Web Server software with the LockFile command, or other Unix type systems. In case of blocking access to certain files, LOCK files of the same name as the original are created, with the .LOCK extension added.