Takeoff/Go-around switches (TO/GA) are switches activated on the autothrottle of an autopilot of a modern aircraft. It has two modes, takeoff (TO) and go around (GA). The switch is the same for both takeoff and go around modes. Usually on approach to land the autopilot will be set to approach mode therefore if the TO/GA switch is pressed it will activate the go around mode. The TO/GA switch is near the throttle levers on Boeings. On Airbuses it is activated by pushing thrust levers fully forward to TOGA detent.
Once an aircraft has lined up, the pilots increase the engines to 60% RPM of Low Pressure Turbine (on Boeings) (50% on Airbuses) (this is done to prevent flame out on take off due to compressor stall) and then press the TO/GA switch (push thrust levers to TOGA or FLEX on Airbus planes), the engines then increase to their computed take off power. Modern aircraft flight management computers will determine the power needed by the engines to take off based on a number of factors such as runway length, wind speed, temperature, and most importantly the weight of the aircraft. In older aircraft these calculations were performed by the pilots before a takeoff. The advantage of having such a system is the ability to reduce wear and tear on the engines by only using as much power as is actually required to ensure the aircraft reaches a safe take off speed.
The go around setting is used when an approach is taking place. If a pilot finds that he or she is unable to land, activating this switch (pushing thrust levers to TOGA detent) will increase the power to go-around thrust. Most importantly, the TO/GA switch disables the autopilot so it does not follow the ILS glideslope anymore, and it does override any autothrottle mode which would keep the aircraft in landing configuration. On Airbus planes it doesn't disengage AP, but causes it to stop following the ILS and perform Go Around maneuver automatically. In an emergency situation, using a TO/GA switch is often the quickest way of increasing thrust to abort a landing. On Airbus planes pushing throttles to TOGA detent does all regarding flight path and speed.
Some aircraft make the TO/GA switch to command the autopilot to fly a missed approach pattern, if the autopilot is programmed to do so. The TO/GA switch can be used even after touchdown if reverse thrust has not been selected.