In a passive sentence what would usually be the object of the sentence is turned into the subject.
Here is a sentence: Tom hit the ball.
The subject is Tom and the object is the ball.
This is called the active voice.
Turn this round so that the subject (the ball) becomes the object and has an action done to it.
The ball was hit by Tom.
This is called the passive voice.
We have just published a new set of pages on rewriting sentences from the active to the passive voice, as well as asking children to explain what needs to be done to change a sentence from the active to the passive.