It is a Krabby Patty that is much larger than the average version and contains double the amount of patties. The Deluxe Krabby Patty is unusually large for someone to swallow the whole thing down and is almost impossible to do.
Role in series
When the sea creatures, SpongeBob, Patrick, Squidward, and Mr. Krabs brag about how sea creatures have the ability to do everything a land creature like Sandy cannot, Patrick brags that land creatures cannot eat a Krabby Double Deluxe in one bite. He then proceeds to swallow the patty. He struggles, as the burger is too large for him to consume, but he eventually manages to succeed. Sandy then tries to eat one to prove Patrick wrong, but cannot because she wears a helmet. Thus, the patty's ingredients splatter all over it.
Mr. Krabs hides his grease from the giant grease trap onto the backyard of the Chum Bucket. Plankton came up with the idea putting grease into his products to add flavor. Plankton's new product attract a long line of customers, and Krabs knew he had to get them back by adding grease to his own food. He informs SpongeBob that from now on, he cooks two patties for every single patty that is ordered and slather the grease of one onto the other. SpongeBob listens to Mr. Krabs when Krabs denies that the grease is unhealthy. The new Deluxe Krabby Patties were able to gain back the Krusty Krab's popularity for a while.
Krabby Patties have already existed with the word deluxe in its name, including the Krabby Deluxe often found on the Galley Grub and the Krabby Double Deluxe from the episode, "Pressure." Despite them existing prior, in "Greasy Buffoons," a customer states, "...something called a Deluxe Krabby Patty?"