Passive voice is one of the most polarizing "bad writing habits" out there. Many self-proclaimed grammar snobs consider passive voice to be incorrect.
Here's the thing: passive voice isn't actually grammatically incorrect. That being said, it's often stylistically better to use active voice.
With active voice, the subject is peforming the action of the verb.
With passive voice, the subject is receiving the action of the verb.
- The book was read by Jamie.
The phrase "by Jamie" tells you who was performing the action of the verb.
Many grammar snobs don't like the passive voice because you can technically construct a grammatically correct sentence without showing who or what is performing the action.
That same sentence could be written as:
That sentence is grammatically correct because it has a subject ("the book") and a verb ("was read"). So why use the passive at all?
Well, there are some instances where passive voice is appropriate. For instance:
If you're not sure who stole your wallet (sorry, that sucks!), passive voice is appropriate because the actor isn't known.