In Spanish, we can use the reflexive to express passive voice.
Whereas active voice makes has an actor and an object, the passive voice only has an object:
Active: I write letters
Passive: Letters are written
Active: I wrote letters
Passive: Letters were written
In Spanish, we can use two constructions for passive voice (and they work in every tense).
The most common is to use the reflexive form, which works like this:
Active: Yo escribo cartas
Passive: Las cartas se escriben
Active: Yo escribí cartas
Passive: Las cartas se escribieron
Notice that the object becomes the subject of the sentence for passive voice constructions, so the verb agrees with cartas here.
The less common passive voice construction mirrors the English construction:
Las cartas son escritas.
Las cartas fueron escritas.