Motion plus.. Its the wii addon that supposed to increase the sensitivity of the wii remote's gyroscopic angle. In other words, if the response of the tilt/angle ect of the controller isn't accurate or fast enough this should improve upon that without having to buy a whole new controller or console.
I'm not sure 100% of the specs, but i'm the guy who spent $ on buying the lightsaber sword sleeve, the blaster looking attachment, and driving wheel when all those items were nothing more than molded plastic that the controller sits into