What you say when you are in a mediocre restaurant, and the person you are with wants to order something very complicated. The theory is that while the restaurant probably does pretty well with simple things, if you try to go beyond their range (even if the entree is on the menu, and looks appealing), you will realize that you're not at the Ritz-- you're at the 99.
person2: I think I'll have the penne pasta with the white wine sauce, roasted cherry tomatos, asparagus, and broccoli. And for dessert, creme brulee. That sounds good, doesn't it? What are you having?
person1: A burger with fries. KISS.