Pronoun-antecedent agreement occurs when a pronoun and the word to which it refers, known as the antecedent, match in terms of number and gender, when applicable. To be grammatically correct, pronouns and antecedents must agree.
An antecedent may precede or follow the pronoun. If the antecedent is singular, any pronoun referring to it must be singular. Likewise, if the antecedent is plural, pronouns referring to it must be plural. If the antecedent has a specific gender, singular pronouns that refer to it must have the same gender. Plural pronouns do not show gender.
An example of pronoun-antecedent agreement is in the sentence "The teacher gave Joe his test." "Joe" is the antecedent for "his," and both words are singular in number and masculine in gender. Another example is "The children opened all their gifts." "Children is the antecedent" to which "their" refers, and the two words are both plural.