This means that you should avoid fatty foods, and also foods with too much sugar in them. Be careful, don't fall into the trap of eating lots of powerbars unless you're on a long ride, or else they'll just make you fat. The small amount of sugar in both foods is also helpful. If sugar from your food can get into your bloodstream fast enough, your muscles can use it to create energy, thus sparing those precious glycogen stores. An even more efficient source of sugar is Gatorade and other sports drinks.
Sugar is complicated.
If you consume too much, it can make you sick. If you consume it dissolved in water, your body can absorb it more quickly. If you consume it before you ride or during the first 30 minutes, apparently it doesn't spare your glycogen at all, but messes up your blood sugar level. And for some reason that I don't understand, you can't rely on sugar for all your energy needs you can use it to spare muscle glycogen, but not to replace it. Drink before you're thirsty.