Matter Muncher Lad is the alter-ego of Chester, created when Timmy wished that everyone had super powers. His powers include super teeth that are capable of eating through anything.
Chester was turned into this superhero form when Timmy wished that everyone on Earth had superpowers in the episode "The Big Superhero Wish!". Chester's abilities in particular included supercharged braces that had drills on them, and the ability to tunnel through the ground.
He wears a black costume and a yellow lightning bolt on the front. His teeth can turn into drills and can (as stated above) eat through anything. His thing is to shout "Munch" over and over (When he's eating, been blasted by F's or having his mind read).