This sounds more complicated than it really is. In a passive sentence what would usually be the object of the sentence is turned into the subject. For example, here is a sentence:
Sam hit the ball.
The subject is Sam and the object is the ball.
This is called the active voice.
Turn this round so that the object (the ball) becomes the subject and has an action done to it.
The ball was hit by Sam.
This is called the passive voice.
We have just published a great set of pages which explain the passive and active voice and ask children to change sentences from the active to the passive and vice versa.