When the brake lamp light comes on, it means that one or more of the brake light bulbs in the rear of the car have stopped working. If you have another person step on the brake pedal while you stand behind the car, you probably will see which lamp (or lamps) doesn't work. Fixing it is usually just a matter of installing a new bulb. The bulbs can often be accessed and changed from inside the trunk. There is usually a plastic panel that has to be removed, to reach the bulbs.
I hope this helps,