Light is on when the parking brake is engaged; if the parking brake is not engaged, it indicates brake malfunction.
It is NOT safe to drive. Stop driving, pull over to the side and check if brake fluid level is correct.
Usual problems: driving with handbrake engaged, low brake fluid level, brake pads worn out
Brakes are the most important parts of your car. A typical brake system consists of disk brakes in front and either disk or drum brakes in the rear connected by hydraulic system to the master cylinder.