ABS = anti-lock braking system
With this feature, the brakes will “pump” automatically. The pedal will pulsate against your foot. This causes some people to ease off the brake, but you should just hold tight. If you do, the car can stop up to 20% faster than without the feature.
Without ABS, the brakes can lock and the tires may stop spinning before the vehicle stops moving. This is dangerous (in an uncontrolled skid) because you have no control over the car from this point on.
You lose the ability to do some trick driving with ABS, but most people wouldn’t do those stunts anyway.
There are ways to learn how to react in a skid – many of them are counterintuitive. If possible, try to learn what to do and practice it (in an empty parking lot, please). I’m glad I did.