From the Super Mario Wiki
The Footstool Jump is a move in Super Smash Bros. Brawl, Super Smash Bros. for Nintendo 3DS / Wii U, and Super Smash Bros. Ultimate that is executed by having the character jump while just above an opponent's head, and can be done up to five times consecutively before landing. The move allows for characters to meteor smash their opponent downward while using them as a spring.
Several mainstream Mario games with a co-op multiplayer mode, such as New Super Mario Bros. Wii, New Super Mario Bros. U, New Super Luigi U, Super Mario 3D World, and Super Mario Maker 2, have a similar move, allowing a player to use another player as a Trampoline.