Pound is the name of Jigglypuff's Neutral B attack in SSB and Forward B attack in Melee. It is a short-range attack that sends Jigglypuff forward a short distance with her arm out in front of her to punch. Pound is very useful when recovering, as it can be used between jumps to give Jigglypuff more horizontal distance. Quickly pressing Up after performing the Pound gains Jigglypuff a slight amount of vertical distance. This technique is known as the Rising Pound. You can also do a "falling Pound", by pressing Down instead of up.
Although the Pound is mostly used for recovery and movement purposes, it can also be utilized as an effect attack. The Pound is a useful move that has a hitbox that stays out longer than one would expect, causing opponents to drop their guard to early at times. In certain situations, the Pound can also serve as a set-up for Jiggly's powerful Rest attack.