It's on factory projectors and TYCs. It's basically a plastic piece that blocks the upper part of the projector beam. When on low beam, the shutter is down, which you can see as a cutoff in the upper part of the beam. When you switch to high beams, the shutter retracts via a solenoid, allowing your low beam to shine unobstructed. With the extra light from the low beam combined with the high beam bulb, it really throws some light.
Low beam= projector with shutter down
High beam= projector with shutter up + high beam bulb