Riboflavin is short in more diets than you'd guess: research published in The Lancet Global Health estimates around 55% of people fall below the recommended intake, high for a vitamin most people never think about.
Its main job is energy production, and it also helps you use several of the other B vitamins, so it plays a supporting role across the group. When intake stays low over time, that's associated with cracks at the corners of the mouth, a sore mouth or tongue, and some skin problems.
Adults need around 1.3 mg (1,300 mcg) a day, from dairy, eggs, lean meat, green vegetables, and fortified grains. There's no established upper limit.