The Somersault(or Crouch Jump in Super Mario 3D Land) is a move that was first introduced in Super Mario 64. It allows the player to jump higher than a regular jump to reach higher places. It can only be used by Mario, Luigi and Yoshi. There is two types of Somersaults: the Backwards Somersault and the Side Somersault.
The Backwards Somersault can let the character to flip in midair and move backwards when the player crouches and jump. It will also allow them to Ground Pound after performing the Somersault. In Super Mario Sunshine, Mario isn't able to crouch as he can only do it when he sprays water, then lets go of the pump while jumping. Luigi can Spin Jump on his way down after performing the Backwards Somersault in Super Mario 64 DS.
The Side Somersault can only be useful when they are trying to change their jump direction. It will allow them to reach higher ledges. Mario only needs one direction and turn around to perform the Side Somersault in Super Mario Sunshine. Bowser would use this in Super Mario Galaxy in the second battle. In Super Mario Galaxy 2, Yoshi can perform this type of Somersault unlike in Super Mario Sunshine.