Mis en scene is one of the more complex and essential ideas of film. It primarily deals with what's shown on screen. Its literal translation is "placing on stage."
Mis en scene differs from filmmaker to filmmaker and project to project, but it's aim is to tell the film's story by a visual means. The director makes a choice by how they want to achieve this by what they put on screen at certain times.
In this still from Sidney Lumet's Dog Day Afternoon, Sonny in the bank vault with the manager and the teller. The bars and the gun make the shot claustrophobic.
In contrast, here is another shot from Dog Day Afternoon. While it is outdoors, Lumet still maintains the claustrophobic motif by having the street crowded with policeman and reporters, and the harsh lighting makes it seem less than inviting.
This shot from Alejandro Jodrowsky's The Magic Mountain has a surrealist bent. Jodrowsky does this by juxtaposing uncommon items (such as the nude servant and the cello, the alchemy apparatus next to the nude man, etc.), but also the bold colors make this scene pop to eye and call attention.