We all know rock, paper, scissors, so I won't explain how that works. If your character attacks with a rock element and the opponent is a scissors element, it will do "sensational" double damage; if the opponent is a paper element, it will do "pathetic" half damage.
rnRemember that these "sensational" and "pathetic" attacks only appear if the attack itself has an element associated with it; many attacks have no element at all. Check out the full Morty list
(unless you're reading it now) to see what every Morty's element is before you start switching Mortys out in battle.