From the Super Mario Wiki
A Volt Shroom is an item that appears in the Paper Mario Series. In Paper Mario it is a yellow Mushroom with a slightly lighter yellow lightning bolt pattern on its "face". The Paper Mario: The Thousand-Year Door variant is black with yellow lightning bolts in place of the usual white spots.
In both games, Volt Shrooms electrify Mario for a short time, causing most enemies to take damage when they physically touch him. It also allows Mario to Jump on electrified foes without taking any damage.
In Super Paper Mario, it again electrifies the player, but instead of damaging enemies, the player could run into enemies and paralyze them without taking any damage. When used, they had to press the A, 1, or 2 buttons as shown on the screen. Each button correctly pressed rose the meter up for a longer effect. Pressing the wrong button dropped the meter down a little. The effect can last anywhere from 25 to 40 seconds, depending on how full the meter gets.
Paper Mario Recipes
Names in other languages
||portmanteau of "volt" and champignon, which means mushroom
||literal translation (possibly from Alessandro Volta, an Italian scientist)