Passive voice is a grammatical construction of sentences which inverts sentence structure from the active sense of verbs to a passive form. It's generally considered to be bad grammar, unless there's a good reason for the form of expression in terms of syntax or other contextual or specific qualifiers. The common sense of active voice is that t'e subject acts upon the agent: In passive voice, the object is acted upon by the subject.
Examples of Passive Voice:
Active: The cat sat on the mat.
Passive: The mat was sat upon by the cat.
Active: Pigeons eat bread.
Passive: Bread is eaten by pigeons.