When you switch the sentence around such that the subject is being acted upon.
Here's what a sentence with active voice would look like:
John kicked the ball.
In this sentence, John is the subject, and he's kicking a ball. The subject is acting upon an object.
Now, what would happen if we made the ball the subject instead?
The ball was kicked by John.
This is passive voice. The ball is the subject, and it was acted upon by an object, in this case John.