A and an are indefinite articles, which means they do not refer to a specific thing. For example, if I say "I want to eat an apple," I am not talking about a specific apple. I could be talking about any apple.
Here are some more examples of how the definite article the is used:
* I went to the store. (I went to a specific store.)
* I saw the man. (I saw a specific man.)
* I read the book. (I read a specific book.)
Here are some more examples of how the indefinite articles a and an are used:
* I bought a new car. (I bought a car, but it is not a specific car.)
* I saw a bird. (I saw a bird, but it is not a specific bird.)
* I ate an apple. (I ate an apple, but it is not a specific apple.)
As you can see, the is used when you are talking about something specific. A and an are used when you are talking about something general.