Internal combustion engines are complex beasts. They run by burning fuels like gasoline or diesel, but they need electricity to do that. That's why every vehicle has a battery, but how does the battery stay charged? That's part of what the alternator does.
The alternator is an electric generator that is turned by a belt when the engine is running. The battery provides the initial voltage to start the engine, and then the alternator takes over. This is how your car recharges its battery continuously while it's running. The alternator provides electricity to the engine...