A lock is a temporary protection you set on a document version when you check it out, to prevent other users from updating it before you check it in. You must have at least the Lock Versions right to lock a document.
Only you or a user with the Full permission on the document can check in the document. If another user tries to check it in, a message warns him that the version is locked. Generally, you will release the lock when you check in the document.
You can also lock documents in the repository without checking them out.
Locked documents display a lock icon in the repository browser:
