If the toyota camry battery light stays on, what is the problem?

The most likely cause for the battery light to come on and stay on in a Toyota Camry is going to be a fault somewhere in the charging system. The charging system includes the battery, the alternator and the connections between the two. The first thing You want to check is the battery connections, make sure they are not corroded or loose. After that You will want to make sure Your battery is still good. The simplest way to do this is stop by any auto parts store, they will usually have a tester and will check it for free. If Your battery is good the only thing left to look at is the alternator but You will need to have a mechanic check that.