Passive voice, per se, is not a grave error to fix or always avoid. It may be the correct construction for the communication goal or context.

Active voice. The subject of the sentence does the action.

Example: Billy made many mistakes.

Passive voice: The subject of the sentence is acted upon but the actor is unknown.

Example: Many mistakes were made.

It would be more helpful (perhaps) or interesting (perhaps) in this case if the party responsible for the mistakes were named. In some cases, however, the actor is truly unknown. In other cases, the actor is irrelevant and the passive voice may have more impact.