The charging system light will come on briefly when you turn on the ignition, but the engine is not running, as a check to show you it is working.
It should go out once the engine is running. If it stays on, or comes on while you are driving, you may have a problem with the charging system. It could indicate that you have problems with a generator drive belt, or another electrical problem. Have it checked right away. Driving while this light is on could drain your battery.
If you must drive a short distance with the light on, be certain to turn off all your accessories, such as the radio and air conditioner.