For me it was because a small sensor that detects wheel movement went bad. Cost about $60 to replace.
The other poster is right though, could be a lot of things. Take it to a shop you can trust and you might get away cheap.
Remember, even though your brakes will work fine with the light on you will not have the ABS function. This means your wheels can lock-up under hard braking or slippery conditions. If you drive in rain or snow you should get it fixed.
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