What is the noun that is replaced by a pronoun?

The noun that a pronoun replaces is called its antecedent.

An antecedent can be a noun or a pronoun.

Examples:

When George got to 19th Street, he got off the train. (the noun "George" is the antecedent of the pronoun "he.")

You and I can finish this if we work together. (the pronouns 'you and I' are the antecedents of the pronoun 'we')