what does it mean if this light comes on while you are driving?

it means either your brakes are faulty brakes system or your handbrake is on.

Anti-Lock Braking System (ABS)

How To Check Your Anti-Lock Braking System (ABS)

Get going 30 MPH or better and slam on the brakes, if the pedal pushes back and/or you hear a growling noise, the ABS is working