I think the Start/Stop that Mark is referring to is for the engine "Eco" management. If the Eco start is switched on, when you approach a junction or join a line of traffic, if you press and hold the foot break once the car stops, so does the engine. Once you release the foot break the engine restarts itself and off you go. I had this function on My discovery 4. The only problem is of course, its no use off road and can be annoying on road at times, especially in slow moving traffic. If you switch the engine start/stop button to off, it disables the engine stopping when you apply the foot brake. Normal routine engine stop start is through a key, alongside the steering wheel.
I think the ECO engine stop is a requirement to meet emission standards in some countries. It also reduces the road fund (Road Tax) annual cost here in the UK if its factory fitted to your vehicle.