What does it mean if your wheels start to skid during braking?

When wheels lock up and skid during braking, the vehicle loses both braking effectiveness and steering control. A skidding tire produces less stopping force than a tire that is rolling at the threshold of grip, and a skidding front tire cannot steer at all because the tire is sliding rather than rolling. On vehicles without anti-lock brakes, the technique is 'stab braking' or 'controlled braking': brake as hard as you can without locking the wheels, and if the wheels do lock, release the brake pedal momentarily to let them roll, then reapply. On vehicles with ABS (anti-lock brakes), apply the brake firmly and steadily; the ABS modulates brake pressure to prevent lockup automatically. ABS reduces stopping distance on slick surfaces and preserves steering capability.