Brake Warning Light
The vehicle brake system consists of two hydraulic circuits. If one circuit is not working, the remaining circuit may still be able to stop the vehicle. For normal braking performance, both circuits must be working.

This light should come on briefly when the engine is started. If it does not, have it repaired so that it will be ready to warn you if there is a problem.
With the ignition on, the brake system warning light comes on when the parking brake is applied. The light stays on if the parking brake is not fully released. If it remains on after the parking brake is fully released, there is a problem with the brake system. Have the brake system checked immediately.
If the light comes on while driving, a chime sounds. Pull off the road and stop. The pedal may be harder to press or may go closer to the floor. It may also take longer to stop. If the light is still on, have the vehicle towed for service. See Towing the Vehicle.
Test yourself
A quick GMC Terrain quiz: every answer comes with an explanation and a link to the manual page.