Pronoun-antecedent agreement refers to the use of a pronoun that matches its antecedent in number, person, and gender. The antecedent is the person or thing that the pronoun refers back to.
For example, in the sentence “David came by; he was looking for you,” the personal pronoun “he” agrees with its antecedent “David” because it is:
Singular: David is one person
Third-person: David is not the person speaking or being spoken to
Masculine: David is a man
An agreement error occurs when the pronoun doesn’t match the antecedent in one of these ways—most commonly in number. For example, “Some men came by; he was looking for you” doesn’t make sense because the singular “he” doesn’t match the plural “men.” The plural “they” is needed instead.