most likely your fluid is low, and most likely this is caused from the pads/rotors being warn. So for now top off the fluid, and plan to do a brake job in the near future.
mine is doing the same thing. so i tried unplugging the connector from the master cylinder and the light went off so that mean that the solenoid in the master cylinder is bad so you'll have to place the whole thing
mine started doing this as well, and now its just always on unless I floor it. I've even had my brakes replaced since it started happening. I have no idea why it's always on, but it sure is annoying!