On a 7-9 you hold 3 burn, and mark a condition.
On a miss, you hold 2 burn, and mark three conditions (like Afraid, Angry, etc)
Burn makes some Moves easier. You could still achieve the same effects without a burn. Overcharge, for instance, is like the 'Unleash your powers' Move, but spending 2 burns gives you an automatic 10+ on your roll. Shielding is like a 'Defend' Move, but uses Freak instead of Savior. Some Nova flares give you entirely new options, like 'Elemental awareness' (ANY question about the world is less limiting than 'Assess the Situation').