Passive sentences are reversed sentences where the subject receives the action instead of performing it. They can be understood best by contrasting them with "active" sentences. Let's take a look at the following two sentences:
Los niños odian a las brujas.
The children hate the witches. (active)
La brujas son odiadas por los niños.
The witches are hated by the children. (passive)
In the first sentence, los niños is the subject. They are the ones who perform the action. In the second sentence, las brujas is the subject, but they don't perform the action; they receive it. Let's look at more examples:
El terremoto destruyó el pueblo.
The earthquake destroyed the town. (active)
El pueblo fue destruido por el terremoto.
The town was destroyed by the earthquake. (passive)
El tigre se comió el águila.
The tiger ate the eagle. (active)
El águila fue comida por un tigre.
The eagle was eaten by a tiger. (passive)
NOTE: Passive voice is always a combination with the verb ser in different tenses, never with the verb "estar".