It either means that the battery juice is running low or the sensor is bad. If you have an Autozone, they can run a test for you for free.
What Does The Battery Warning Light On Dash Board Mean?
It means that the alternator is no longer being charged. You can drive like this for a short while, perhaps a few hours, then the battery will be discharged and your car will not start.
So, the alternator needs to be replaced.
So, the alternator needs to be replaced.