The word 'a' is classified as an indefinite article. This means it doesn't refer to a specific, identifiable entity but rather any member of a group. 'A' is used to introduce a singular noun (or pronoun) when the exact identity of that noun isn't crucial to the meaning of the sentence. It's about introducing something non-specific.
The article 'a' is employed in various contexts.
Introducing a noun for the first time. 'I saw a dog in the park.'
Referring to any member of a group. 'Can I have a cookie?'
Describing a noun in a general sense. 'She is a teacher.'
I bought a car last week. (It could be any car, not a specific one.)
He is a good listener. (Describing a quality of someone in a general sense.)