This part of your Control Panel is used for administrating your website's pages as well as organizing them in the main menu. On the right you will see a list of your pages. On the left are several admin tools which allow you to add new pages as well as edit, rename and delete your current ones. To update your menu you can simple use the drag and drop functionality. To do this first click on a page in the tree menu below, now without letting go of the mouse button you can move the item and place it under another menu item by dropping it there. When you move menu items like this, just click the Save Menu Changes when you're done to update the main menu. To use the rename, edit or view tools, simply click on the page first to select it, and then click the appropriate tool.
Please watch the video below for a quick demonstration on the Page & Menu Admin tool, just click the play button in the middle! (Total time: 6 min. 9 sec.)