What does flame one do in minecraft?

Its an enchantment that goes on a bow, and when you shoot a mob/player with it on it sets them on fire

On a bow it is essentially identical to the Fire Aspect on a sword. Arrows you shoot from a Flame 1 bow are on fire, and set mobs so struck on fire. They can also set off TnT, but will not set fire to any other block type. Note that Nether mods are immune to this effect (save regular skeletons).