A Ram Air Turbine (RAT) is a small turbine that is installed in an aircraft and used as an alternate or emergency hydraulic or electrical power source. The RAT generates power from the airstream based on the speed of the aircraft and is connected to an electrical generator or to a hydraulic pump.
In extremely rare instances when airplanes lose power, the ram air turbine deploys from the airplane’s wing or fuselage and rotates to extract sufficient power from the airstream to control and land the aircraft.
As you may have saw in a recent tweet from Infinite Flight, of a 3D rendering of the B777, most notice that there may or may not be a ram air turbine on the rendering seen here. It has not yet been confirmed to be present or functional.