Din's Fire is a special attack that can be performed by Zelda. It is done by pressing Forward B. It is an attack with considerable range, and when it hits your opponent, it sets them on fire and hits them up. It can be controlled with the control stick (while holding B) and can be released by letting go of the B button. When done in the air, it sends the user into the "helpless" falling animation. It can be countered or deflected, but this will not send the projectile back at the user. It can also be absorbed by Ness' PSI Magnet.
Due to the long wind-up and wind-down lag of this attack, the key to its successful use is prediction and precise timing. It is mostly used to prevent opponents from rushing in at Zelda or as an edgeguarding technique.
It is based on the Din's Fire spell from The Legend of Zelda: Ocarina of Time. Although, rather than being a small sphere, it is a large explosion of fire radiating outwards from Link, used primarily as an offensive spell, but also used to solve puzzles.
Zelda's Din's Fire in Brawl appears to function similarly, but moves and detonates a lot faster, making it a legitimate projectile rather than a laggy, prediction-only attack.