The words anything, everything, and something are indefinite pronouns. Each of these pronouns can be used in place of a noun.
I can’t find anything in my messy closet.
The word anything takes the place of a noun that you might have used in that sentence.
I put everything into that box.
Once again, the word everything takes the place of a noun that could have been in that sentence.
I need to get something to eat.
The pronoun something takes the place of an unknown noun.
The indefinite pronouns anything, everything, and something are all used in place of a specific noun.
▶Now you try it. Replace the noun in this sentence with one of the indefinite pronouns on this page. I have a book to give you. Did you replace the noun book with an indefinite pronoun? Good for you!