The ABS modulator pump motor runs only when the ABS is active. When the system dumps pressure to prevent a skid, the fluid flows into accumulators. The pump motor then runs to push this fluid back against the high pressure of the master cylinder. This keeps the brake pedal from sinking to the floor.
If this motor burns out, the ABS cannot recycle fluid. The system will disable itself and turn on the warning light. You can test the motor by triggering it with a bi-directional scan tool.