Fire Orb is a special attack featured in Super Mario RPG: Legend of the Seven Stars that is exclusive to Mario. He'll learn this Special Attack once he reaches level 3. It costs 5 FP, though can deliver a hefty amount of damage based on how fast you can press the Y button.
To launch the attack, just quickly mash the Y button as much as possible; each time the player does, a fireball will be launched from Mario's hand and into the enemy, each one becoming progressively stronger. Like all fire based attacks in the game, Fire Orb is useless against enemies that are composed completely of fire.