ARTICLES 'A' AND 'AN'

What is an article?  Like adjectives, articles modify nouns.
English has two articles: THE and A/AN. the word 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 = definite article
a/an = indefinite article
Let's try to put it this way, if I say, "Let's read the book," I mean that book, a specific book.
Let's read the book about Peter Pan
If I say, "Let's read a book," I mean any book (for instance whatever book you can find on your bookshelf or anywhere else) rather than a specific book mentioned above.
Let's read a book which is on the Library.
Here's another way. The word THE is used to refer to a specific or particular member of a group. For example, "I just saw the students of 5th level student of Kareaga dancing on the street." There are many students there in that school but only one particular batch is there on the street. Therefore, we use the.