The "waste gate" is a bypass of the turbine housing. If it's open, a part of the exhaust fumes is led directly into the exhaust to prevent the turbo from spinning too quickly and delivering too much boost.

The flap that is opening and closing this bypass is operatated by the rod of the waste gate actuator. A spring inside the actuator head keeps the rod tensioned towards the actuator head and the waste gate closed. If air pressure is applied to the head of the actuator however, the rod is pushed away from the actuator head against the spring pressure, opening the waste gate.

In a simple setup the air pressure necessary to open the waste gate is taken directly from the compressor wheel and is therefore equal to the boost.

"-31" is only the ending of the parts number of an uprated actuator commonly used. It will have a stronger spring inside.