From the Super Mario Wiki
The Superball is a type of projectile weapon found in Super Mario Land. Superballs are very comparable to the Fireballs of several other Mario platformers (including Super Mario Bros.). In Super Mario Land, after collecting a Flower (and thus transforming into Superball Mario), Mario can shoot Superballs by pressing . Superballs, as stated above, are very much like fireballs, as they can damage enemies (although, unlike fireballs, Superballs sometimes take multiple hits to defeat even generic enemies). The Superball differs from the fireball in one way in that they, rather than bouncing up and down on the ground, bounce directly off of walls at a right angle. This can sometimes make it difficult to hit enemies, as the Superball can bounce into the air before it reaches them. Also, the Superball has the convenient feature of collecting Coins for Mario, something that the Fireball lacks. Every Superball disappears approximately four seconds after being shot, unless it hits an enemy earlier. Mario can only shoot one Superball at a time.