A taillight is the red light on the back of a car, truck, or bicycle that helps other drivers see the vehicle, especially at night or in bad weather. When you turn on your headlights, your taillights usually turn on too, glowing red to show drivers behind you that there's a vehicle ahead.

Taillights on many vehicles become even brighter when you press the brake pedal, signaling to other drivers that you're slowing down or stopping. This brake light feature helps prevent rear-end collisions by giving drivers behind you time to react. Many vehicles also have amber or red turn signals built into the taillight assembly that blink to show which direction you're turning.

The red color isn't random: red light travels well through fog and rain, and humans associate red with “stop” or “caution.” If you've ever ridden your bike at dusk, you might have clipped a small red taillight to your seat or backpack so cars can spot you from behind. A broken taillight is both dangerous and against the law in many places, which is why police officers sometimes pull drivers over to warn them or require a fix.