Flex mode refers to folding the device at a certain angle, and making it stay in a fixed position. It was made possible by the foldable form factor and its hardware that allows angular folding. The upper part of the screen becomes the main viewing area, while the lower part becomes the control area. This format is especially useful for the camera, video calls, and media streaming.
A "Fold" type device's Flex mode supports two situational modes that are used according to the folded angle. First, if you would like to comfortably enjoy videos or other media content in the wide aspect ratio, you can fold the phone at an angle less than 90 degrees and use the cover screen. On the other hand, if you would like to simultaneously enjoy videos and control other features or perform other tasks with split screen view, you can unfold the device at a wide angle and continue using Flex mode on the main screen.
In a "Flip" type device, the Flex mode allows splitting of the main screen into upper and lower areas. (Compare in the screen below)