This shows you all module positions. When discussing CMS earlier, these module positions then will be the content in your database.  Your 'design' is a separate element and sits 'behind' these modules. These modules are 'blocks' of content, or various features (for example, a login form etc). This means that your content has great flexibility and you can publish your content in any position on the page that you want, very, very easily. All the modules are fully collapsible. This means that if there is no module published in a certain position, this module position will contract and disappear, allowing your page look 'normal' and as intended, without huge gaps of 'white space'. 

Left aligned

Right aligned