The Leather Ball is a somewhat circular ball made out of many pieces of leather stitched together. This ball can be thrown to the ground for a dragon to chase and return to its trainer. It is like playing a variation of "fetch" but with dragons.
At some point before the events of School of Dragons the Leather Ball was created by Hiccup in order to help trainers play and bond with their dragons.
During the events of the game, Hiccup gave the ball to the player once they had obtained their first partner dragon so that they could play with their dragon and create a closer bond with it.