What does the ABS warning light mean?

The ABS warning light (the letters "ABS" lit up) comes on when there's a fault with the anti-lock braking system.

Ordinary braking usually still works fine. What you lose is the anti-lock function itself, which normally stops the wheels locking up and skidding under hard braking - especially relevant on wet or icy roads.

It's a reduction in a safety system rather than a loss of brakes altogether, so it's not usually treated as an immediate stop-the-car emergency the way an oil pressure or coolant light is - but it's more than cosmetic, and it's worth getting checked reasonably soon.