What does an bulb do?

Answer 1

When a pedal is depressed like the brake pedal, the switch under the pedal activates the circuit and sends electricity thru the corresponding bulb (in this case the brake bulb), causing it to light up.

Turning the buttons on the stalk behind the steering would activate another circuit and cause the lights to come on in the dashboard, with a further click causing the headlights to come on.

Depending on the colour of the glass of the bulb, or the colour of the enclosure casing the corresponding light is emitted.