In passive sentences, the subject receives the action of the verb.  In active sentences, the subject does the action.  However, the meaning of both sentences is the same.  The passive form is only possible with transitive verbs and is much more common in English than in Italian.  The passive form consists of the verb essere plus the past participle of the main verb followed by da (by) and its contractions.  Essere should be in the same tense as the verb in its corresponding active sentence.  The past participle agrees in gender and number with the subject.

I contratti sono firmati dalle ragazze.  The contracts are signed by the girls.
La stanza è stata arredata da Carlo.  The room was decorated by Carlo.
L'affito sarà pagato dai miei genitori.  The rent will be payed by my parents.