Backflips, also known as Backward Somersaults, are jumps Mario can do that causes him to move upwards and backward. You can perform a backflip by pressing and holding Z and pressing A. Holding forwards during the backflip can cause Mario to have a net forwards movement. The Vertical Speed (VS) value that a backflip starts with is 62. With all of the vertical speed combined, you can get 512 units up from where you started the backflip.
The C^ Backflip trick allows you to backflip facing any direction without having to turn first, saving a few frames in some TASes.