The narrator interrupts the story to comment, judge, give an opinion, or speak directly to the reader.
In The Master and Margarita
Who told you that there is no true, faithful, eternal love in the world? Then cut out the tongue of that infamous liar!
The narrator interrupts the story to opine and turn to the reader, swearing that eternal love exists and ordering that the tongue of whoever says otherwise be cut out. He promises to prove the thesis with Margarita before any scene.