An object is locked when it is imported in the ITP/MDK Repository or when it is opened for editing. The name of the user who locked the object is shown in the ITP/MDK Repository. This user can edit the object.
Saving the object will store a working copy of the file in the ITP/MDK Repository. This working copy is a private copy. It is only accessible to the user that is editing it. Each time you save the object, your current private copy will be overwritten by a newer version. The original file however, remains locked, and does not get overwritten by the working copy. As far as other users are concerned, the original object is still there. They cannot edit it because it is locked but they can still view it.