They are twelve Krabby Patties on wheat buns. Although not seen when SpongeBob makes the patties, they apparently contain salt, as suggested by John when he asks for extra quantities of it.
Role in episode
John orders them and Squidward writes his order on paper and hands it to SpongeBob alongside the rest of the orders. SpongeBob cooks them by saying "Bun, patty, ketchup, tomato, bun, bun, patty, ketchup, tomato, bun, bun, patty, ketchup, tomato, bun, bun, patty, ketchup, tomato, bun, bun, patty, ketchup, tomato, bun" in a fast tone of voice while adding the said ingredients, and then announces the order by shouting, "One dozen cryin' cows on the farm!"
Squidward takes the order and says "Thanks, Farmer Brown" to him, then proceeding to hand the plate of patties to John. The latter asks for extra salt, but Squidward says that they're all out. John asks if he could double-check for clarification, but Squidward lousily refuses, to John's anger.
Although they contain lettuce, it is not mentioned when SpongeBob fastly narrates the ingredients of the patty.