In short
Anti-lock Braking System — prevents the wheels from locking under hard braking.
ABS stands for Anti-lock Braking System. Under hard braking, ABS pulses the brakes many times per second to prevent the wheels from locking up, allowing the driver to steer while braking firmly.
All cars on UK roads sold since 2007 must have ABS. Locking the wheels with ABS is normal; without ABS (very old cars) you must cadence-brake manually.