La carta fue enviada por María.
English: The letter was sent by María.
The passive voice in Spanish ('voz pasiva') is a way to focus on the action or the object of the action, rather than who does it. The subject receives the action.
Use the passive voice in Spanish when you want to emphasize the action or the result, or when the person doing the action is unknown, unimportant, or obvious.
La carta fue enviada por María.
English: The letter was sent by María.
La casa será construida por los ingenieros.
English: The house will be built by the engineers.
El examen ha sido corregido por el profesor.
English: The exam has been corrected by the teacher.
Las decisiones son tomadas por el comité.
English: The decisions are made by the committee.