Envelope protection keeps the airplane within the "flight envelope". Outside the flight envelope bad things can happen such as loss of control, or even structural failure. Pilots are trained to stay within the envelope but sometimes get close to the edge without being aware of it - for example; letting airspeed slow to the point near an aerodynamic stall. In this case the autopilot would lower the nose allowing airspeed to increase and warn the pilot with an audible message, "Speed protection active."