It's a Wheatstone Bridge installed vertically inside the rotor mast, the varying resistance measured across it being directly proportional to the bending moment applied to the mast. It's a valuable tool which tells the pilot when he is approaching the point of knackering the mast, for example when landing on sloping ground.
It's usually optional because, if slope limits are observed, and correct landing technique employed, the mast should not become knackered anyway.