Stop-start engages when the car stops and you put your foot on the clutch – it turns the engine off when you're stationary and switches it back on when you re-engage the clutch. It is designed to save fuel and tailpipe emissions that would otherwise be burnt when the car is idling (at traffic lights, for example).

It only works when it's efficient to do so, though, usually it'll be inactive as the car warms up, to help it reach its optimum running temperature as quickly as possible. Other strains on the engine might stop stop-start operating, such as having the air-conditioning running on a hot day.