A small jump (commonly short hop) is the act of pressing the jump button and letting go before the character leaves the ground. This will result in a jump that is shorter than the normal jump. This technique can be performed in all three Super Smash Bros. games.
The short hop technique is essential in high level play, allowing the user to perform aerial attacks close to the ground. Short hops are essential for the SHFFLing technique.
Jumping speed can affect the timing of the short hop. In Melee, for instance, a Fox player must release the jump button within 2 frames to short hop. Bowser, by contrast, requires only 7 frames and his short hops are considered much easier. The height of a short hop varies from character to character as well. Also, in Brawl, it's much easier to short hop than in Melee.