From the Super Mario Wiki
Platforms are obstacles in almost every adventure video game, including the Mario franchise. They are physical objects that the player jumps across to get through levels or discover secrets. Mainly in side-scrollers, the player can jump up through the bottoms of some platforms (and in Super Smash Bros.' case, drop through the tops as well), which can't be done in tridimensional games. Platforms often have gimmicky traits, such as moving. These are sometimes referred to as Lifts.