What is the purpose of a spark plug and how do I replace it in an Audi A4?

I recently bought an Audi A4 and I noticed that it needs a spark plug replacement. However, I have no experience with cars and I am not sure what the purpose of a spark plug is or how to replace it. Can anyone provide some guidance?

A spark plug is an essential component in a car's ignition system. It provides the spark that ignites the fuel and air mixture in the engine, allowing your car to start and run smoothly. To replace the spark plug in an Audi A4, you will need to locate the spark plugs under the engine cover, disconnect the electrical connector, and use a spark plug socket to remove and replace the old spark plug. Remember to consult your car's manual for specific instructions.