This light is not only for the E-brake as some people have said.
Your brake fluid is either low or low on pressure. If its low, add brake fluid, if your brake fluid is full then your problem is you're brakes are needing replacing now. Check your front pads, if they are low, thats whats causing your brake light to come on.
Change your brakes and pads if need-be and then add fluid if need-be. Your light should go out.