Protector is the fifth rank in the Dark Brotherhood, which is awarded after completing the requirements of a Acolyte. It is at this point within the Journeymen class that one may wield a basic lightsaber given to them by their superiors.
It is part of the Journeyman class, the fifth step down the path to Dark Jedi Knight.
The Requirements to be promoted past this rank are much more difficult, and are as follows:
The requirements for the promotion from Protector are found at the above link, the completion of the above requirements will earn a promotion to Guardian.