Typically, this is the result of a sensor malfunction (or sending info that there's a mechanical malfunction).
Start with the easy stuff: Check the brake fluid level. If it's low, check your pads. If they're worn, it's time for a brake job, and you're due for a brake fluid flush anyway. Next, check your brake lights. If there's a light out, or an error in the brake light circuit, that'll do it. Then, check your brake light switch at the pedal - if it's acting up, it'll cause the same symptom.
Here's more info:
vdc-off-slip-lights-coming-on-t501963.html