Main Bus Stops
A plain white sign with a red London Transport symbol means the stop is a main stop and all buses will stop there automatically.
Request Bus Stops
Red signs with a white London Transport symbol mean it's a request bus stop. This means the bus will not stop automatically so you have to stick your arm out to 'hail' the bus when it approaches.
If you want to get off a bus at a request bus stop, you need to ring the bell (once only). Make sure you give the driver plenty of notice to stop.
Knowing your bus stops can be helpful when you find out about the regular London Underground Weekend Engineering Works.