Each individual layout represents one of the various page types available to you when designing your web store. The layouts act as a template for your store, allowing you to make adjustments and preview before publishing live.
Each layout is comprised of one or more widgets. They provide the web store structure, each one representing one part of the overall page layout. Widgets contain within them the specific UI functionality to which they relate, for example, the Order Confirmation layout contains information on items ordered, prices, shipping address, and so on.
Upon opening the Design tab you will see the Select Layout drop down menu on the left side of your screen along with the layout configuration toolbar icons displayed on the right side of your layout screen. The toolbar gives you access to the Layout Preview, Grid View, Layout Settings, and Clone Layout configuration menus.