It looks like a regular Krabby Patty, or any other burger, but gigantic in size. The amount of meat in the patty is so great that normal size spatulas cannot flip it.
Role in episode
Squidward says that no one has ordered a Monster Krabby Patty in ages. When SpongeBob tries to flip the patty, Spat (SpongeBob's spatula) is broken in half, and is taken to the hospital. When Spat is cured, another customer orders a Monster Krabby Patty but Spat does not break this time, but instead SpongeBob's arm breaks off.