B6 is the busiest of the B vitamins here, and the one with a real ceiling. Beyond releasing energy from food, it's heavily involved in protein metabolism, in making the brain-signalling chemicals called neurotransmitters, and in forming red blood cells. When intake stays low over time, that's associated with low mood or irritability, a sore mouth, and a weakened immune response.
Shortfalls aren't rare: research published in The Lancet Global Health estimates around 51% of people don't get enough to meet the recommended intake. Adults need about 1.4 mg (1,400 mcg) a day, from poultry, fish, chickpeas, potatoes, and bananas.
Unlike most water-soluble vitamins, B6 has an upper limit worth respecting: it's best not to exceed 100 mg a day, because consistently high doses, almost always from supplements, are associated with nerve symptoms over time.