When navigating to Settings =>Design =>Template Editor, you will be presented with every single template that is potentially used by your store. These templates reside in a special folder of your site (accessible via FTP) that is known as the "Common" folder.
When you install a design theme onto your store, what you are essentially doing is adding a frame.html template file that controls your store's framing layout, and a default.css stylesheet that controls the store's basic look and feel. All other templates are typically served from the common folder. This allows us to push necessary updates and new features to templates to all 3dcart users globally because we are updating only the Common folder.
Clicking the "Edit" button for any one of these templates in the Template Editor will basically create a copy of the template and place it to your specific theme's folder. This way, any changes made to the template affect your site and your site alone.
The disadvantage to this is that any further updates to the template (to add features and correct code) will no longer apply to your store since the template is not being served from the common folder anymore. However, the advantage is that the file becomes unique to your site and can be updated as you please.
Tip: Editing template files via the Template Editor screen is not for the faint of heart. There is no graphical representation as there would be with WYSIWYG editor. Therefore, the template editor is best suited for those accustomed to HTML editing in its raw state.
For a more complete guide to making edits to your store's design, please review the following tutorial which describes the many ways of editing your design in order from easy to more advanced.