Until you've had it explained to you it seems baffling - just stock up on fresh fruit and veg, and maybe some snacks like alpen lights or muller light yogurts, as for drinks maybe get some diet drinks. Its difficult to say because it will also depend on whether you're going to do red day or green days. good luck with SW - you'll enjoy it x
mullerlight or weight watchers yogurts are a must for me, plus some nice fruit and I love raw carrots.
Also quorn sausages etc are a good choice and you can have them on red or green days.