The antecedent of a pronoun is the noun that the pronoun is intended to refer to.
In the United States, an astronaut candidate must be recommended by her previous organization.
--"Astronaut Selection and Training," Compton's Encyclopedia (modified)
Make sure your pronoun agrees in gender and number with its intended antecedent. Make sure the antecedent of your pronoun is clear and unambiguous. See Pronoun Reference.