This begs two obvious questions. What is passive voice, and why would I want to avoid it? (Actually, there's also a third question here. Zombies? What? That will come later.)
Passive voice almost always comes with a form of the word "was". It looks like this:
The ring was destroyed.
Who destroyed the ring? We don't know. All we know is that there was a ring, and someone or something destroyed it.
Here's the same sentence, in the active voice (opposite of passive):
Frodo destroyed the ring.
In this sentence, we know who destroyed the
Passive voice can also happen with a subject, though, usually indicated with a "by ___" at the end.
Passive: The hobbits were taken to Isengard by orcs.
Active: Orcs took the hobbits to Isengard.
In this case, we know who took the hobbits to Isengard. These sentences both say the same thing, but using the active voice makes for stronger and more engaging writing.
Here are some more examples of active vs. passive, so you can see the difference:
Passive: The Witch King was killed by Eowyn.
Active: Eowyn killed the Witch King.
Passive: The potatoes were eaten by Sam.
Active: Sam ate the potatoes.
To do a test for passive voice, take any sentence and add "by zombies" to the end of it. If the sentence makes sense, it's probably passive. Besides, this can get entertaining, depending on sentence.
Aragorn was attacked.
Aragorn was attacked by zombies.
But why would you want to avoid passive voice? After all, it makes sense, doesn't it?
It makes sense, but it also makes for weak writing. Look at the above examples. The active voice sentences are more focused, less wordy, and more, well, active. They are more lively and just sound better overall.
Always use the active voice whenever possible. Well, okay, there are some occasions where you should probably use the passive voice, but since I just spent this whole post talking about why you shouldn't, we'll save it for later.
Here is another great article on passive vs. active, if you want to know more.
PS: Yes, all my examples are Lord of the Rings-related. No, I don't care if you judge me for that. :)