It basically demarks on the plate what portion of which type of food you should eat, with one half being vegetables, one quarter carbohydrates and one quarter protein.
This is actually an excellent ratio for your meals to keep the nutrient content up and the calorie content down (although I usually end up eating all three in equal amounts) and I think the idea behind it is a great one, however I won't be purchasing one myself. I think that eating should be a pleasurable experience and that includes presentation - to me there's just something a bit school/prison catering trayesque about this plate that puts me off - and I certainly wouldn't want to use it in the presence of company!
That said, when you're putting your meals together, keep the ratio of macronutrients in mind. Always try and match the volume of protein with the volume of carbs (most people eat too many carbs relative to proteins) and also eat at least as much veggies as carbs. And for you veggies out there remember that pulses, soya products, tofu and nuts and seeds all count in the protein portion.