This is a term most often used in both production and development. It describes a really important dramatic or comedic scene or sequence in the story – particularly those that plant seeds, provide some sort of payoff, convey character traits, or provide resolution. These scenes are so pertinent to the story that they cannot be eliminated or augmented without affecting the integrity of the film. For example, the opening scene in the THE SIXTH SENSE where Bruce Willis’ character gets shot is an important setpiece – the twist at the end wouldn’t work without it.
Nowadays, you’ll often hear executives use this term to describe complicated or expensive scenes that require a lot of additional planning and money to execute. Examples of this could be scenes with pyrotechnics, car chases, gun fights, heavy stunts etc. These are the sequences that require lots of meticulous planning, storyboarding and allow less room for improvising. These setpieces tend to be the scenes that define the movie – Leo and Kate on the bow of the Titanic, Jack Nicholson busting open the door with the ax in THE SHINING, or ET flying on a bicycle with the moon in the background.