You can edit any object on your page or add new objects using your mouse. To edit existing objects, a single click will allow you to drag and resize objects, while a double click will allow you to edit the contents of the object, e.g. images in the gallery.
When you select an object, the 'Editor' will display a number of panels which allow you to change settings on that object, e.g. for text this could be it's position on the page, e.g. 'Info' tab, or the font that you use, e.g. 'Font' tab. Different objects will have different settings.
The 'Toolbar' at the top of the page allows you to make other changes to your site, e.g. upload your own images/files, or add new pages. The icons provide shortcuts to some of the most common features.