Active sentences have the subject coming before the verb.
e.g. They opened the door.
S V O
He kicked the ball.
S V O
Sometimes however, a speaker or writer may not wish to focus on the ‘subject’ but on the ‘object’ of the sentence.
e.g. The ball was kicked (by him)
The door was opened (by them)
When sentences are inverted in this manner, we say they are in the passive voice.