A word like “he,” “she,” or “it” that stands in for a noun is a pronoun.
Pronouns help speakers and writers avoid repeating the same noun. Instead of saying “Maria found Maria’s book,” we can say “Maria found her book.” The noun or noun phrase that a pronoun refers back to is called its antecedent. In the question’s examples, he, she, and it are third-person personal pronouns.
English pronouns change form according to grammatical role and, in some cases, number, gender, or case. For example, he becomes him when used as an object, while she becomes her. The pronoun it usually refers to an object, animal, or abstract idea, though English also uses singular they when a person’s gender is unknown, unspecified, or nonbinary.
A common mix-up is assuming every short replacement word is simply a noun. Pronouns form their own part of speech, while conjunctions join words or clauses and adverbs modify verbs, adjectives, or other adverbs. Some words traditionally called possessive pronouns are also analyzed as determiners, depending on the grammar system.