The "Long Description" appears below the image, in a tab that can be set to show "Reviews" first, or "Description" first. Tabs can save screen real estate by hiding the text until a person clicks the tab to see what's revealed below it. Us this long description to fill in more details about the product, but more than just telling about features, you need to talk about benefits. Get the viewer involved by projecting how it's going to benefit them. Don't just bore them with features of the product as they've probably already thought about the more obvious ones. Try to come up with interesting things they might not have already known.