Bounce is the angle of the sole of the club that hangs below the leading edge of the wedge.

If you look at the bottom of your sand wedge, you will see some extra tilting down and away from the leading edge of the front of the club face.  The bottom of the club is shaped differently to help it glide through the turf or sand without digging in too deeply.  The back edge of the sand wedge is the bounce, striking the turf or sand first so that the club skips instead of digging in.