The phrase comes from traditional film sets, where crew would switch on the lights, start the camera rolling, and the director would call action to begin a take. It became a pop culture shorthand for show business through decades of movies, cartoons and behind-the-scenes parodies. Online, creators reuse it as an instant framing device for entrances and reveals, much like the directed by Robert B Weide ending turns a moment into a joke scene. Its meaning needs no explanation, which is why it still works for short edits.