From the Super Mario Wiki
A Plunger, also known as a Plumber's Helper, is a tool typically used to release stoppages or clogs in the drains of appliances such as sinks, toilets, bathtubs and showers. Mario and Luigi, being plumbers, frequently use plungers throughout The Super Mario Bros. Super Show!, and to a lesser extent, in The Adventures of Super Mario Bros. 3. A plunger consists of a rubber cup with an attached stick "shaft", usually made of wood or bronze.
Throughout the course of The Super Mario Bros. Super Show!, it is revealed that Mario and Luigi almost always have a plunger on hand, which they often use as a projectile or a bludgeon. They also use plungers in lieu of wedges allowing them to climb up walls, or keep from falling down. By tying a rope to a plunger, it can also be used as a grappling hook.