The setting should almost always be Letterbox. Letterbox scales the video to fill the screen without altering the aspect ratio. The setting is persistent. You do not need to change it for every movie/show you watch.
Original displays the original resolution. If you play a 1920x1080 video on a 3840x2160 screen, you will see a 1920x1080 video in the middle of the screen with black bars on all sides.
