Prevent other people from modifying a file.

The lock operation is used to get exclusive rights to modify a file. Not all version control tools include this feature. In some cases, it is provided but is intended to be rarely used. For any files that are in a format based on plain text (source code, XML, etc.), it is usually best to just let the VCS handle the concurrency issues. But for binary files which cannot be automatically merged, it can be handy to grab a lock on a file.