When a sentence uses the active voice, the subject of the sentence performs the verb as in the following sentence:
The boy kicked the ball.
Here we see that the boy is performing the action of kicking.
When the subject of the sentence has the action performed to it, then, that is what is called the passive as in the following sentence:
The ball was kicked by the boy.
Here we see that the subject, the ball, was receiving the action/verb of being kicked.