I'll start with active voice because it's simpler. In an active sentence, the subject is doing the action. A straightforward example is the sentence "Steve loves Amy." Steve is the subject, and he is doing the action: he loves Amy, the object of the sentence.
Another example is the title of the Marvin Gaye song “I Heard It through the Grapevine.” "I" is the subject, the one who is doing the action. "I" is hearing "it," the object of the sentence.