A Clavis is a special silvered vambrace, a piece of armour granted to Space Marines upon becoming a Deathwatch Keeper. Created during the Dark Age of Technology, the workings of the clavis are not fully understood by the Adeptus Mechanicus, but it is known that it interfaces with the Space Marine's nervous system and monitors his vital signs. The clavis acts as a unique and complex key that contains a myriad of Inquisitorial override codes and other, more arcane systems that allow it to bypass nearly any technological seal. 
The clavis communicates with light, vibration, and other, less-known means to unseal magnetic locks and shield barriers at the Keeper's command. Servitors and other autodefences register the Keeper as a friend and stand down in his presence. The clavis is what allows a Keeper to walk his vigil undisturbed and reach nearly any secure zone.