Small Jumping, the official name for the more commonly-called Short Hopping, is the act of pressing the joystick upwards, or the X, or Y buttons very quickly, and letting go before the character leaves the ground. This will result in a jump that is shorter than the normal Full Jump. This technique is essential in high level play, and is primarily seen in the form of SHFFLing.
Some characters are harder to short hop with, as they jump very quickly. Fox has a 4 frame time period in which the player needs to let go of the jump button, and is thus considered difficult. Bowser has a 9 frame period, and is much easier to short hop with.
The height of a short hop varies between each character. This technique can be performed in all three Super Smash Bros. games.