Din's Fire is a magic spell from The Legend of Zelda: Ocarina of Time, given to Link by a Great Fairy in a cave near Hyrule Castle. While the other two magical abilities in the game, Nayru's Love and Farore's Wind, are used for defense and travel respectively, Din's Fire is an offensive spell used to attack enemies. Once cast, Link will toss a ball of fire at the ground which expands and surrounds him dramatically harming enemies in the radius. Of the three magic spells, Din's Fire is the only one that isn't optional as it's required for lighting a ring of torches to enter the Shadow Temple.
Din's Fire is an attack performed by Princess Zelda in the Super Smash Bros. series, debuting in Super Smash Bros. Melee and later returning in Super Smash Bros. Brawl for the GameCube and Wii respectively. The move is Zelda's side B move, which when executed will cause Zelda to release a ball of flames. You'll be capable of controlling where it goes, though not entirely. Once the ball is close enough to one of your opponents, you'll be able to press B to make it explode.