It is used to refer to a noun that is not something or someone specific such as a pen, an apple, a bus).
" A" or "An" is used when it is not important as to which the nounis refering to (example: an apple on the table, a pen on the table or a book from the room).
It is unknown as to which the noun is referring to (example: a teacher in the hall, a letter in the mail box, a car in the street).
"A" is used if the letter before the next word starts with a constenent (not a vowel a, e, i, o, u).
An is used if the letter for the next word starts with a vowel (a, e, I, o, u).
Examples:
I see a dog in the street. *
The girls see the big black dog on the other side of the street. **
Please bring me a pen.*
Please bring me the black pen that is on my desk.**
I think that is a teacher from the school.*
I think that is the math teacher.**
* A refers to something that is not specific, it is not clear or important as to which dog, pen or teacher.
** The refers to a specific pen "the black pen" not just any pen, "the big black dog" not just any dog, and "math teacher" not just any teacher.