A sideflip is a type of jump that can be executed while Mario is moving and provides him with a decent amount of height. It is performed by quickly reversing direction and then pressing the jump button.
Sideflips have a height of 496 units, giving them an identical height to both backflips and vaults. This height is fixed, meaning it is not determined by how long the jump button is held. In addition to gaining height, sideflips also move Mario laterally by a small amount in the direction that the stick was reversed to just before the jump.
Due to this lateral movement, sideflips are useful in very similar situations to backflips, which follow a similar path. One such example would be to clear an overhanging ceiling. The key difference is that sideflips are sometimes slightly better for situations where Mario is already moving prior to the jump.