The "voice" of a verb has to do with whether or not the subject of a sentence acts upon something else (as agent) or whether the subject is acted upon (by an agent).
There are two voices in English: the passive and the active:
The verb is active, when the subject (agent) does the action (verb) to something (object).
The verb is passive, when the subject takes the action upon itself.
- Active: John ate the apples.
* John, the subject, is the one performing the action of the verb.
- Passive: The apples were eaten by John.
* The apples, the subject in this sentence, are receiving the action of the verb.