When your battery light comes on it could mean that your battery is going bad and not holding enough charge, or your alternator could be going out and not providing enough amperage to keep everything running in your car plus charge your battery up.
To put it simply when your battery light comes on it's an indicator of a charging and voltage problem.
Sometimes it's possible that the reason your battery light is on is due to a broken alternator belt but other times it's due to a faulty battery, loose connections to the battery, or even your alternator not putting out enough charge.