Voice

Voice is a grammatical term which is used to tell whether the subject of the sentence is acting or receiving the action expressed by verb.
When the subject is acting, we say that the subject is the doer. When the subject is receiving the action, we say that the subject is receiver.

Jane washed the window. (Jane is the doer of action)
The window was washed by Jane. (Window is the receiver of the action).

In the grammar we say that the verb in the first sentence is in the active voice because the subject is the doer, or is doing the wshing.
The verb was washed is in the passive voice because it represents the subject of the sentence as the receiver of the action. In other words, the subject is not acting, but is passive. The doer, or the actor, appears in a phase introduced by the preposition by.
The verb in the passive voice is never a simple verb. It is always a verb phrase.