Plain English Meaning
The lowest speed at which a twin-engine airplane can still be flown straight if one engine quits. Slower than this, the working engine pushes the airplane sideways more strongly than the rudder can correct, and the pilot can no longer keep it pointed where it's going.
Full Definition
The slowest airspeed at which a multi-engine airplane can be controlled directionally with the critical engine suddenly inoperative, the remaining engine at takeoff power, and the airplane configured per the manufacturer's certification conditions. Below this speed, the rudder and ailerons can no longer counteract the yawing and rolling forces produced by asymmetric thrust, and the pilot loses directional control.