From the Super Mario Wiki
Pressure Plates are objects in Mario & Luigi: Superstar Saga. They are encountered at Chucklehuck Woods, Woohoo Hooniversity, and Joke's End alongside the Wall Faces, with each location changing its appearance. When one is stepped on, a nearby Wall Face fires a projectile blast at whoever is standing on it. If they are inside of a Barrel, the blast is reflected back. The Pressure Plates are only activated if Mario or Luigi step on them, though this also includes when Luigi is inside of a barrel.