Sometimes you don't want documents be updated - for example once the minutes of a meeting have been signed off. To do this, you can "lock" the files. Once you've done this, a small padlock icon will appear next to the document:
... and that means only Admins for the group can change or overwrite those documents.
Documents uploaded by Admins automatically start off as "locked" and you can unlock them by clicking on the small padlock icon on the right-hand side of the screen next to the relevant file or folder.