Expert Mode is a new setting introduced in the Definitive Edition that can be toggled on and off at will. It's not an actual difficulty setting that makes enemies stronger or have more health, but it rather acts like an experience bank for your characters. To access Expert Mode, which is available from the very beginning of the game, press the (+) button to open up the settings menu and navigate down to the Expert Mode listing. Upon choosing it for a first time, you will receive a brief tutorial on what it does and to actually activate Expert Mode, push in the right analog stick.
With Expert Mode on, you are able to adjust the levels of your characters at any time you want within this menu. Each character has their experience bank and as you earn more experience, it will be added to this pool for you to use later. The reason this was likely added in is because playing the game normally and doing everything as you go (discovering locations, completing sidequests, etc), it was really easy to become overleveled for basic enemies in new areas. With Expert Mode enabled, you can fight enemies around their intended level and earn a lot more experience than fighting them at a higher level.