Motion smoothing is a picture processing feature, found under various brand specific names, that analyses the incoming video and generates extra frames between the original ones to make fast movement look smoother and reduce motion blur. It works by estimating how objects move between two frames and inserting a calculated frame in between.
While motion smoothing can genuinely reduce blur during fast sports action, it is also well known for giving film and television drama an unnatural, overly smooth look often described as a soap opera effect, since it removes some of the subtle motion judder that is normally part of how film content is shot and shown. Many viewers prefer to turn it off, or use a lighter setting, for movies while keeping it on for live sport.
Filmmaker Mode is a standardised picture setting that automatically disables motion smoothing and other processing to preserve a film's original look. See our guide to TV picture settings for how to adjust it for different content.