When the subject is acted upon by the verb, this is called 'passive voice'.
Not sure how to identify passive voice? Here's a good rule of thumb: If you can add “by zombies” after a verb - it’s passive voice.
|The house was haunted [by zombies]||The haunted house|
|The game was released [by zombies] last week||Nintendo released the game last week|
This article goes into more detail on how to use the "zombie test" to improve the readability of your article.