It is the number one most important thing you can do to start on the road to fitness. In laymen's term, it's paying attention to what you eat each and every day and sticking to nutritious, low-fat foods that fuel your body. It's watching the size of your portions, watching your calorie and fat in-take, eating the recommended servings of fruits and vegetables, whole grains, drinking lots of water, and just staying away from the junk. I live by a saying that I read on another forum, and keep it posted in my kitchen on the fridge: "You can exercise til the cows come home, but if you don't clean up your eating you'll just be a cow coming home". (Of course there's a lovely picture of a cow on it, too, to drive the point home.)