The term derives from theater.
Literally, it’s the French language word for “putting on the stage.” (“Scene” is French for stage.)
In film, it means: everything that’s in front of the camera, not including what the camera adds by its framing, angle, etc.
In short: the mise-en-scene of a movie is everything in the theater that could be put on a stage: sets and costumes, lighting, props, etc.
Mise-en-scene can also mean: the process of staging the action by the actors, the ‘business’ they perform (e.g., opening a door, looking through a drawer, etc.)