Could be many reasons but in any case have a trusted mechanic inspect those brakes NOW! before you have an accident.
The abs light is a separate system from the brakes.. I know that sounds funny but the abs system is after the break system. Your brakes will still work if the abs is inop. The brake light is usually a fluid level indicator in the master cylinder, not a light to tell you to replace the pads or shoes. I would look for a fluid leak or a problem with the parking brake.
Your brakes are failing
check fluid level
Light is usually comes on, when there is a problem with pressure in brake lines. Inspect the brake fluid level, and check for leaks.
your park brakes are on
fix your brakes
As long as the red " BRAKE " light is not on in your dash you still have " normal " brakes even with the ABS light on
You probably need to change your back brakes.
In most new cars the parking brake light is also activated when the linings of your brakes wear out.
Do a diagnostic check and it should shoot you a code out to you.It should be only a brake sensor it for wheel speed
You have stepped on the brakes
Is it the parking brake light or the ABS light? is it red light or amber?