Voice is a grammatical term which
indicates the relationship between a subject and verb. There are three different
voices in Spanish (and English): active (what you might call normal),
reflexive, and passive.
In the passive voice, the action
described by the verb is being done to the subject by an agent.
Reparo los coches.
I repair the cars.
Ella lee el libro.
She is reading the book.
Todo el mundo le respeto a él.
Everyone respects him.
Mis amigos quieren a mi madre.
My friends like my mother.
Los coches son reparados por mí.
The cars are repaired by me.
El libro es leído por ella.
The book is read by her.
La comida es cocinada
a las ocho.
The meal is cooked at 8 o'clock.
Él es respetado por todo el mundo.
He is respected by the everyone.
Mi madre es querida por mis amigos.
My mother is loved by my friends.
The agent (the person performing the action on something else) is
introduced by the preposition por.