To “deck” a Stratocaster-style vibrato tailpiece is to pull it tight against the top of the guitar by tightening the claw springs in the back of the guitar. A primary reason to “deck” a bridge is so that it stays in tune if a string breaks. Some players suggest that decking the tailpiece the vibration transfer through to the body, adding to sustain and resonance. And, some feel that it affects the string tension and playing feel of the guitar, especially when bending notes.
Note that when a Strat tailpiece is decked, it can only be used to lower the pitch of the strings. It can not be pulled back to raise the string pitch.