Occasionally in battle, you will notice various things in the background, ranging from trees in the field to rock formations in caves. These seemingly innocuous background objects can be destroyed and release an item.
It seems that the various objects react to specific ranged attacks. While Surf may work on one, it won't work on another, and other moves such as Earthquake will not effect the same object Surf does.
Some of the known moves that work are Air Cutter, Twister, Razor Leaf, Pedal Blizzard, Surf, Rock Slide, and Heat Wave.