What is passive voice?
In a passive sentence, the thing acted on comes first and the actor is pushed to the end or left out: “the report was written”, “mistakes were made”. In the active version the actor leads: “Sam wrote the report”.
Passive sentences are longer and vaguer. Readers have to work out who did what, and sometimes they cannot.
When passive voice is fine
Use it when the actor is unknown or does not matter: “the bridge was built in 1890”. Use it when the thing acted on is the real subject of your paragraph.
Spare marks passives so you can check each one, not so you can remove them all. It is cautious on purpose: phrases that only look passive, such as “she was tired” or “the office is located in Leeds”, are left alone.