A Jelly Patty was one of the items on the Krusty Krab menu. It is just a Krabby Patty with Jellyfish Jelly in it.
History
After Fred told Mr. Krabs how much he liked SpongeBob's new recipe, Mr. Krabs made it an item on the menu. After SpongeBob and No Name saw how Mr. Krabs was hurting the jellyfish, they forced Krabs to take it off of the menu.