What the devil is pre-vis, or pre-visualisation? Not wanting to completely rip the Wikipedia page and paste it here, the simplest way of explaining it is "a function to visualize complex scenes in a movie before filming." Pre-visualisation is most often used in film production - the Star Wars films used it to create some of their most iconic scenes - this clip about using footage from old WW2 fighter films is a great example...
moddb.com/news/pre-visualisations-in-game-development
