The number plate light is located on the back of the vehicle above the number plate/registration plate, and serves to illuminate the number plate in the dark. Ensuring that your number plate is visible at all times is a legal requirement and allows police to use the details of the number plate to complete any necessary checks on the vehicle, so it’s important that the number plate is kept clean and in good order.
It’s worth noting that in poor visibility, other travelling vehicles may only be able to see your vehicle is there by noticing your illuminated number plate, so care should be taken to ensure that the number plate light is free from dirt and grime so that it can illuminate the rear number plate sufficiently, and the correct bulb should be fitted and working.
Your first indication that a number plate light has failed could be the light failing to light in the evening. This is usually rectified by changing the bulb, but if this does not correct the problem it could indicate a problem with the wiring supplying power to the light, and should be fixed as soon as possible.