The wheel speed sensor
A very common cause. Each wheel has a sensor reading its speed. A dirty, damaged or failed sensor turns the light on.
The wire and connector
The sensor harness runs near the wheel: it can break, chafe or corrode at the connector, especially with winter road salt.
Other possible causes
A damaged or rusty tone ring, low brake fluid, a fuse or a fault in the ABS module can also turn the light on.
How to narrow it down
An ABS-capable code reader shows which wheel is involved. Then replace that wheel's sensor or harness and clear the code.
If the red brake warning light is also on, do not drive: that is a different problem affecting the braking circuit.
This advice is general. If in doubt, have your brakes inspected by a mechanic before driving.