This application transforms the active form into passive and the passive form into active.
A verb is said to be active when the action is performed by the subject.

Luigi eats an apple.

eat = active verb, as there is an action of Luigi who takes the apple and eats it.

A verb is said to be passive when the action is not performed by the subject but is suffered by the subject.

The apple is eaten by Luigi.

is eaten = passive verb, as the subject is the apple, which undergoes the action of being eaten in a passive form.

The property that distinguishes active and passive verbs is called the verb form.