In analog filmmaking, a zero cut is a method of making rolls of film (usually negative) by overlapping multiple frames. This makes it easier to change shots without having to make and register an entirely new roll. This method is used specifically for blow-up. When blowing up a print, every shot is given frame handles so that the registration pin of the printer is never engaging with a splice. It is most commonly used when blowing up a 16mm print to a 35mm format. Only cut your movie to the correct length if it's absolutely necessary. If the lab can do it only with an optical print, which is more expensive than contact prints, it's not worth it. It's a little more complicated to cut the film to the correct length, so the negative cutter charges more for it.
The term zero cut can also refer to a type of wipe (transition) where an extra or an object"wipes the frame" by walking in between the action and the camera, blocking the audience's view for a moment. During that wipe, a new shot is introduced by reversing the movement at a new location/time. Zero cuts are a bit of a trick in filmmaking. Instead of cutting the scene perfectly, the director will simply cut the scene and then shoot a new scene at the end of the cut. This is called a "zero cut" because it cuts the scene and replaces it with another scene. The scene will always be exactly the same, but the viewer doesn't know that.