Usually it means that the brake fluid reservoir is low.
This can mean one of two things usually.
- Normal brake pad and rotor wear has allowed the brake pads to move closer to the rotors and pull fluid into the cylinders. This is normal but infrequent and usually toward the end of brade pad life, it usually is fixed by topping off the reservoir with fresh brake fluid, if the brakes are not worn past the point of safety.
- You have a brake system leak. You have no idea if it is a big leak or a small leak. A small leak can be tolerated for a short time but a big leak can result in total loss of braking power so the safe and wise thing to do is have it checked immediately after it comes on.