The light is telling you that the anti lock part has a fault - the brakes will work perfectly under normal braking, you just don't have the anti-lock function.  You may also lose the load sensing/apportioning function as well, since this operates through the ABS on many modern cars and anyway it's an MOT failure so get it fixed as soon as possible.  Most likely cause is a wheel speed sensor.  If your mechanic doesn't know this, I'd find a new mechanic to investigate and fix it.

Tim