Battery Warning Light
Just as certain children's toys need batteries to operate, so does your car. If a toy's packaging says, "batteries not included", then if you don't supply the batteries, the intended child recipient is likely going to disappointed because his toy won't work. It will just be dead.
Similarly, if your car's battery fails, your car can die. Your car contains a rechargeable 12-volt battery that powers all of its electrical components. These include the car's engine control computer, ignition system, headlights, radio, power seats, widows and locks, among other parts. So clearly, the batter is vital to your car.
We all know that batteries don't last forever, at least not on their own. Any battery will eventually die unless it is recharged. Rechargeable household batteries are common now. When they die, the owner simply places them in their charger and plugs the charger into the wall. A few minutes later, the batteries are fully charged and ready for use.
Your car's battery is no different. It actually has its own built in recharging system, which include the alternator and a voltage regulator to keep the battery charged and provide electricity to the engine while it is running.
The battery indicator light on you car lets you know something is wrong with your car's charging system. It could be a broken alternator belt or a failing alternator altogether. Of course, even with you car's battery charging system, it doesn't last forever. Batteries are rated with how many years or miles they can expect to last, and when a battery begins to fail, it simply won't take a charge at all.
We've all experienced this. We get inside of our car and turn the ignition, and it won't start. Either the engine turns over and over without the car starting, or nothing happens at all. In this case, you can generally just remove the battery from the car, take to an auto parts store and purchase a new battery. You can even generally get a prorated discount on the new battery, if the one you are bringing failed earlier that it was rated to.