Niacin helps turn food into energy and keeps your nervous system and skin healthy. When intake stays low over time, that's associated with tiredness, skin problems, and digestive upset.
It's less commonly short than some of the other B vitamins: research published in The Lancet Global Health estimates around 22% of people don't get enough to meet the recommended intake. Adults need about 16 mg a day for men and 14 mg for women, from meat, poultry, fish, peanuts, and whole grains.
Niacin is one of the few water-soluble vitamins with an upper limit worth respecting: it's best not to exceed 35 mg a day. That ceiling is really about supplements, since high doses can cause an uncomfortable flushing of the skin, and the amounts you get from food stay well below it.