What does "indefinite articles" mean in English?
There are only two Indefinite articles in English they are: "a" and "an".
Indefinite articles imply ambiguity, in other words when you use an indefinite article you are not referring to a specific object (a person, place, thing, or idea). Let's take a look at a few examples to better understand this concept:
I could say something like:
Can you give me a cookie, please?
In this case I don't care what cookie you give me, it could be chocolate chip, oatmeal raisin, or any other cookie, I just want a cookie. The direct article in this sentence is a.
Or I could say something like:
Could I have that cookie, please?
In the sentence above I'm referencing a specific cookie, let's say its the chocolate chip cookie, and I do care which cookie I get. In this case since the word "that" is referring to a specific object it is called a definite article (I'm telling someone that I want something specific).
For pronunciation reasons we use the indefinite article "an" when the next word begins with a vowel, that is, a, e, i, o, u, or a "silent h". And we use the indefinite article "a" when the following word begins with a consonant (any letter that isn't a vowel) or if the word begins with u or eu (here's a trick: if it sounds like "you" use "a").
I hope this helps!