The Super Mushroom is a standard Mario item. When released, it moves like a Goomba. If Mario touches a Super Mushroom, he becomes Super Mario, doubling in height and gaining the ability to break Brick Blocks above him by hitting them. If Mario is hit while he is Super Mario, he gains a few seconds of invulnerability and becomes normal Mario again.
A Super Mushroom would normally be placed inside a Question Block, but Mario Maker allows you to place them in any situation.
Dropping a Super Mushroom on an Enemy will modify that enemy, usually making them bigger. Shaking an enlarged enemy will cancel this effect. Dropping a second Super Mushroom on an enemy who is already enlarged will play an amusing Easter Egg but has no effect.