We use the when we are thinking about a specific thing. Compare a/an and the:
1. Tim sat down on a chair (perhaps one of many chairs in the room)
2. Tim sat down on the chair nearest the door ( a specific chair)
If I say, "Let's read the book," I mean a specific book. If I say, "Let's read a book," I mean any book rather than a specific book.
A is used before a noun that starts with a consonant sound (e.g., “s,” “t,” “v”). An is used before a noun that starts with a vowel sound (e.g., “a,” “o,” “i”).