Voice is a grammatical term that describes the relationship between a subject and a verb. There are three voices:

  1. Active voice - John kicked the ball.
  2. Passive voice - The ball was kicked by John.
  3. Pronominal voice - John kicked himself.

The active voice is the most common: it indicates that the subject of the verb performs that action.

Mon ami a vendu la maison. - My friend sold the house.
This verb is active - the subject (mon ami) performed the action of selling la maison (the direct object).

In the passive voice, the subject of the verb undergoes the action being performed.

La maison a été vendue par mon ami. - The house was sold by my friend.
This verb is passive - the subject (la maison) underwent the action of selling and mon ami was the agent. The emphasis changes from mon ami to la maison.

The pronominal voice indicates that the subject is acting upon itself, which is very common in French, much less so in English. 

La maison s'est vendue très vite. - Literally, "The house sold itself very quickly," but in English, we'd say The house was sold very quickly.
This verb is pronominal: it makes no mention of who sold the house, instead focusing on the simple fact of the house be sold.

Voice exists in all tenses:

Active Passive Pronominal
Les jumeaux lisent le livre.
The twins read the book.
Le livre est lu par les jumeaux.
The book is read by the twins.
Les jumeaux se lisent.
The twins read to each other / to themselves.
Le maire ouvrira la nouvelle usine.
The mayor will open the new factory.
La nouvelle usine sera ouverte (par le maire).
The new factory will be opened (by the mayor).
La nouvelle usine s'ouvrira demain.
The new factory will open tomorrow.
Les parents avaient déjà contacté l'école.
The parents had already called the school.
L'école avait déjà été contactée (par les parents).
The school had already been contacted (by the parents).
Les parents s'étaient déjà contactés.
The parents had already contacted each other.
Let me take a look at that...