With Locks|Lock you can lock the selected files in the repository. You can enter a Comment, why you are locking these files.
The option Steal locks if necessary will lock the requested files regardless of their current lock state (in the repository). In this way it can happen that you 'steal' the lock from another user, what can lead to confusion, when the other user continues working on the locked file. Hence you should use this option only if necessary (e.g. if someone is on holiday and has forgotten to unlock important files).
Keep Update to HEAD before selected to perform an update to HEAD. Only the latest revision of a file can be locked.