Let’s answer the most basic version first. “Continuity” is the illusion that allows cuts in a movie to appear seamless, when actually the two shots we see may have been filmed hours, days, or even months apart. For example, one character throws a punch, and there’s a cut to a wider angle in which the punch lands. In the second cut, we may not even be looking at the same two people; the actor getting punched in the face may have been stealthily replaced with a stunt performer, with the shot angled to hide their face. It’s a grand magic trick with a whole crew of magicians collaborating to pull it off.

Perhaps because our brains are used to seeing continuous action in real life, they’re fairly easily fooled into thinking that the action in one shot was immediately followed by the action in the next shot. Most of the time, we’re not even consciously aware of cuts as we’re watching a movie. As editor Walter Murch (“Apocalypse Now”) wrote in his book “In the Blink of an Eye,” the ideal cut “should look almost self-evidently simple and effortless, if it is even noticed at all.” He describes the ease with which humans adapted to cuts in movies as a kind of miracle:

The mysterious part of it, though, is that the joining of those pieces … actually does seem to work, even though it represents a total and instantaneous displacement of one field of vision with another, a displacement that sometimes also entails a jump forward or backward in time as well as space.

Murch goes on to explain that certain cuts have a greater tendency to disorient the film viewer, and from this emerged the “rules” of editing. One of the best-known rules is the axis of action, or the 180-degree rule, which dictates that if we’re looking at two characters, the “eye” of the camera must remain on one side of them. If a character is on the left side of the frame and the eye of the camera suddenly jumps to the other side, so that they are now on the right, the result is jolting for the viewer. Suddenly, they are aware of the cut. Murch continues: 

The discovery early in this century that certain kinds of cutting ‘worked’ led almost immediately to the discovery that films could be shot discontinuously, which was the cinematic equivalent of the discovery of flight: In a practical sense, film were no longer ‘earthbound’ in time and space … Discontinuity is King: It is the central fact during the production phase of filmmaking, and almost all decisions are directly related to it in one way or another — how to overcome its difficulties and/or how to best take advantage of its strengths.