A pronoun is a word that takes the place of a noun. An indefinite pronoun means that the pronoun replaces a noun that is not specific. The indefinite pronoun refers to a noun, but the reader or listener is not sure which exact thing is being referenced.
The words anything, everything, and something are all pronouns. Each of these words can be used to replace a noun. However, the noun is undetermined, so these words are called indefinite pronouns. It is not definite to the reader which noun is being replaced by each of these pronouns.
Did you buy anything at the store?
The word anything replaces the noun that you might have bought. We just don’t know which noun that is.
I put everything we need into the car.
The pronoun everything replaces the specific noun or nouns that the speaker is talking about.
Dad is looking for something to cook for dinner.
The pronoun something stands in for a noun.
The words anything, everything, and something are indefinite pronouns. Each of these words takes the place of a noun. We don’t know exactly which noun it is, so we call these words indefinite pronouns.
▶It’s your turn. Find the indefinite pronoun in this sentence. I bought Mom something special for her birthday. That wasn’t too hard, was it?