Hi Marco,
This is a tag which identifies a version (or revision) of a file, or, more often, different versions of a set of files; it is mostly used to associate them to a specific patch, build, branch... so that they can later be identified/retrieved using this user-defined name.
Each versioned file can be associated to one or several labels, however each label can only be associated to a given revision of a specific file.
In brief, this is a must-have feature of any version control system.
When you get or check out a file, you can either specify a revision number or a label.
I hope it's clearer.
Cheers,