English has two articles:
the and a/an. The is used to refer to specific or particular nouns; a/an is used to modify non-specific or non-particular nouns. We call the the definite article and we call a/an the indefinite article.

For example, if I say, “Let’s read the book,” I mean a specific (definite) book. If I say, “Let’s read a book,” I mean any book. Tip: Think of it as though a is short for any.

We use an before a word that starts with a vowel to make it easier to say (an apple, an egg, an impossible idea, an octopus, an unusual use).