The aerial receives radio waves so that you can listen to the radio in your car.
Aerials are usually mounted onto the outside of the car but some cars have an aerial mounted internally. Internal aerials are usually fixed to the dashboard of the car. The aerial is connected to your car stereo by an aerial cable which houses the conductor needed to receive radio reception, and is essentially an extension of the center conductor inside the aerial cable.
There are various types of aerials available, such as manual aerials (retractable and non-retractable) including pillar mounted and roof mounted; electric aerials, which can be extended fully or retracted back into the body of the car, which is a useful function making it less likely to be damaged by corrosion due to the weather or broken off accidentally; and satellite aerials, which tend to be more expensive but the quality and reliability of reception is of a high standard. There is also an aerial sat nav which is used to receive a signal and send this to the sat nav. Each type of aerial has its benefits, so it's a good idea to give it some consideration before deciding which type of replacement aerial is right for your car, making sure it is compatible with your car, though it is possible to buy an aerial adapter if needs be.
The aerial may, of course, get damaged or become misaligned, or simply over a long period of use will stop working properly. If your aerial snaps then you will, of course, need to buy a new one. If your aerial is still intact but you are experiencing poor reception and sound quality is getting worse over time, then this is a sign that you need to replace your aerial. It is possible that your car will still be able to receive FM radio when the aerial has stopped working but if your car is unable to pick up a signal on the AM frequency then it is very possible the aerial is broken.