Indefinite articles are used when we are referring to an unspecified thing or quantity. We use them when we don’t know (or don’t care) which thing we’re talking about.
There once was a sheep.
Since I don’t know which sheep it was—that is, I don’t know its name, where it’s from, or anything about it—I can’t say the sheep.
How to Use the Indefinite Articles A vs. An
The two indefinite articles in English are a and an.