What part of grammar indicates an unspecified person or thing?
An indefinite article adjective (a or an) indicates an unspecified person or thing.
If I said, "Hand me a pencil." The "a" in the sentence signals that I will accept any pencil, hence unspecified. However, a definite article adjective indicates a specified person or thing (the). "Hand me the pencil" is a different sentence because "the" indicates a specific pencil.