The answer is nothing, as long as it goes off shortly after starting your vehicle. If it does not, that indicates that your electrical system is not charging properly.
This can be caused by one or more components in your electrical charging system. The three most common are the battery cables, alternator/regulator, and the battery.
But first the important thing to remember is that you will not damage your vehicle while operating it while the battery light is on. It will simply stop running when the battery runs out of juice. To delay this as long as possible, do not run any unnecessary electrical components such as a radio, AC or heater fan.
You can take the vehicle to a mechanic, or you can start checking it yourself.
First check the battery cables. Are they corroded with a white powdery substance. If so, this can be cleaned with a steel brush. Then check to see if they are loose. The posts and clamps should also be cleaned with steel brush. The clamps should be pushed down as far as possible on the posts and tightened with a wrench.
Then check your alternator belt. If it is not broken, loose or damaged, and the light is still on, then it is time to take it to the repair shop.
Here they will check your alternator and regulator to see if the alternator is sending juice to your battery. If this checks out all right, they will then put a meter on the battery in order to see if it is charging. If your battery is more than three years old, it probably needs to be replaced.
At this point the light should turn off. If not, it most likely indicates a fault in your electrical wiring system. This requires an experienced auto repair professional. Your mechanic will either do this himself or recommend a shop that specializes in electric problems. Call an auto repair shop to have your vehicle looked at today!