What does it mean if a vehicle’s brake lights are on?

When a vehicle's brake lights are illuminated, it typically signifies that the driver is applying the brakes, which generally indicates that the vehicle is slowing down or coming to a stop. This serves as a critical communication tool for other drivers, alerting them to the vehicle's intentions and helping to prevent collisions.

In contrast, the other options do not accurately capture the purpose of the brake lights. For instance, when a vehicle is in reverse, it will usually activate a different light feature known as the reverse lights, which are distinct from brake lights. Similarly, a parked vehicle should not have its brake lights on unless the driver is actively engaging the brakes, and turn signals are separate indicators that show the intent to turn rather than braking action. Thus, the presence of the brake lights is a clear indicator of slowing down or stopping, making this answer the most appropriate choice.