Voice refers to whether the subject performs or receives the action of the verb, with active voice indicating the subject performs the action and passive voice indicating the subject receives the action. Both active and passive voice are explained through examples, and reasons for using each are provided such as active voice being more direct while passive voice can emphasize the recipient of the action or not reveal who performed it. The document also covers forming passive voice verbs using a form of "to be" plus the past participle.