Question

what are passive voices?

Answer 1

The passive voice is a grammatical construction (specifically, a "voice"). The noun or noun phrase that would be the object of an active sentence (such as Our troops defeated the enemy) appears as the subject of a sentence with passive voice( the enemy was defeated by our troops).

Answer 2

Passive voice is used when the focus is on the action.
Example: My bike was stolen.
            : A mistake was made.
            : A letter was written.