We can change the emphasis in a sentence by changing the sequence. If the agent of the action comes first, we say that is active voice. In the sentence The scientists launched the rocket yesterday, the agents are the scientists and the rocket is the object (i.e. what is acted upon).
However, we can reverse the order and put the object as the subject and place the agent at the end: The rocket was launched by the scientists yesterday. The emphasis is now on the rocket; the theme of the sentence has changed. That is obvious if we realise that we can even drop the agent of the action: The rocket was launched yesterday. That sequence is passive voice.