When you need to work on a document for some time, you don't want other users to edit it at the same time and disturb your own work. This is typically the case where you lock the document by clicking the icon .
A locked document remains visible to other users but only the locker and users with Manage everything permission can edit it. Other users can see the document locked through the icon . A tooltip indicates who locked the document and when.
Users with Management everything permission can unlock document on other users' behalf so a document does not remains locked during the locker's vacation and blocks other user's work, for instance.