This applies to high jumps. High jumps are performed either by jumping while Mario moves at running speed or, when stationary, by holding Up on the d-pad while jumping. (A similar effect is also observed when jumping out of water.) If you release the jump button before reaching the peak of Mario's regular jump height you will be able to jump higher than normal - which we call superjump. A detailed graph with what button press duration translates into what jump height can be found here: https://docs.google.com/spreadsheets/d/1LqGVXif8ZDI4dDwWFwfGWQ4P_siRlP2ZE3kvXMCE3ig/edit?usp=sharing