The form that a verb takes when the subject receives the action is passive. In passive constructions the doer of the action is supressed for the focus to be on the action itself. For example, an active sentence would go as follows:
I played a song yesterday
The passive sentence would be "a song was played yesterday". "The song" is the one that has been played, it is the one which receives the action. Therefore, the form that the verb takes when the subject receives the action is passive.
