What is the passive voice and is it bad?
In the active voice, the subject does the action: the committee rejected the proposal. In the passive voice, the subject receives it: the proposal was rejected by the committee — or, with the actor dropped entirely, the proposal was rejected.
The grammatical marker is a form of to be plus a past participle: was rejected, is being reviewed, had been decided.
It is not bad. It is overused, which is a different complaint. Blanket advice to eliminate it produces worse writing, because there are cases where the passive is the correct choice:
When the actor is unknown or irrelevant. The building was completed in 1890. Naming the builders adds nothing.
When the recipient is the real subject. She was awarded the prize keeps the focus on her, which is usually what the sentence is about.
In scientific writing, where convention favours the samples were heated over I heated the samples, though that convention is loosening.
When you deliberately want to soften attribution. This is also why it earns its reputation: mistakes were made is the classic evasion, and your account has been closed avoids saying who closed it.
So the useful question is not whether a sentence is passive but whether hiding the actor serves the reader or the writer. If it serves the writer, rewrite it.
A common misdiagnosis: not every was is passive. She was tired is simply a description. Grammar checkers frequently flag these wrongly, so treat their suggestions as prompts to look, not instructions to change.
A useful diagnostic doing the rounds is the "by zombies" test: if you can add by zombies after the verb and the sentence still parses — the proposal was rejected by zombies — it is passive. It is not rigorous, but it catches most instances faster than parsing the grammar.