Great question! Ali has done a fantastic job in answering your question.
A passive sentence is the opposite of an active sentence.
The boy kicks the ball is an active sentence. Here, you have a subject (the boy) followed by a verb (kicks) and an object (the ball). Active sentences have this structure of subject-verb-object.
In a passive sentence, this is reversed.
The ball is kicked by the boy.
:) I hope this makes it even clearer than Ali's already great explanation!