A phrase which waiters, waitresses and other such order takers are required to say if you order a Coke in a restaurant that only serves Pepsi. This serves a very important purpose: preventing trademark dilution. If you ordered a Coke, and you were served Pepsi, then that would imply that Coke is the same thing as Pepsi. Because it is not, you must confirm that yes, you really do want a Pepsi, even though you said "Coke."
Of course, if you're one of those people who actually prefers one over the other, you would be better off knowing that you will be receiving Pepsi instead. I have never actually seen someone explicitly order a Pepsi, unless they had already heard "Pepsi okay?" from that same restaurant in the past. Coke is it.