Is this a real company? Is it like you are getting a bite to eat with them? I had something sort of like that once, with a small company, at a mexican restaurant. I think it was to get to know my personality and see how I was in an informal setting. Mostly they asked about my hobbies and my personal background. They asked a couple interview-y questions but it wasn't serious, they weren't looking at my resume taking notes or anything. It was like eating lunch with a couple of strangers. I was uncomfortable, didn't like the food, and kind of bombed it / wasn't all that charming. This was for a job that was probably more client relations than I was suited for. That was the most 'informal' interview I've had.
IF yours is anything like that (maybe I'm way off), and you order food, make sure you get something that is easy to eat and that you will be able to finish easily. Whatever stage you're at, prepare for it like it was a normal interview, less focus on the technical side though. It could just be a nice little chat, or they could ask you questions like it's a 1st round behavioral interview (for example).
Good luck!