When the ABS light comes on the braking system on your car reverts back to standard braking. The mostly problem is a wheel speed sensor to have failed. Subaru did have some problems with the ABS pump relay at the front of the engine compartment just behind the passenger front head light but this caused the ABS pump to stay running all the time when the relay stuck.
Check the brake fluid level in the resiviour and get the system checked as soon as you can.
...antly while driving and braking for about 15-30 minutes of driving in traffic. After that, the ABS light will come on in the dashboard and then the brakes will stop pulsating and work great.