What is a noun antecedent?

An antecedent is the noun or pronoun that a pronoun replaces.

The noun or pronoun either precedes the pronoun in the same sentence, a previous sentence, or is known to the speaker and those spoken to.

For example:
When George got to 19th Street, he got off the train.

• "George" is the antecedent of the pronoun "he."

Students in on-line classes have to keep up with their assignments.

• The Word "students" is the antecedent of the Word "their."