When you depress the brake pedal in your car, the pads press against the rotors to create friction. This slows the car down and eventually brings it to a stop. The brake fluid level sensor is located in the reservoir and is responsible for monitoring the level of fluid in the system.
If the brake fluid level gets too low, it will trigger the brake failure warning light. This is because there isn't enough fluid to create the necessary pressure to operate the brakes. As a result, the pads won't be able to press against the rotors properly and the car will take longer to stop.
This is a serious issue that should be addressed immediately. If you see the brake failure warning light, make sure to pull over and check the level of fluid in the reservoir. If it's low, add more brake fluid until the light goes off.