Locking source files prevents someone else from modifying them (in the tip configuration) while they involved in a View Compare/Merge session. Locks apply to the "whole item" within the view regardless of timestamp.
When starting a View Compare/Merge session, you can enforce default locking behavior by setting one or more locking options in the View Compare/Merge Wizard. Once in the session, you can override the lock status of any items.
When you close a View Compare/Merge session, you are prompted as to whether you want to retain the locks created by the View Compare/Merge session.