The thumbnail list shows an overview of all pages in the document.
For instance, above screenshot shows the thumbnails of a document of 6 pages.
Opening pages in the editor
You can click on a page in the thumbnail list to open it in the editor. The current page will be focused (in above example that is page 2).
Moving pages
You can move a page by dragging it to its new location in the thumbnail list.
Copying pages
You can copy a page also by dragging, but then holding down the [Ctrl] key. A small [+] will indicate that you are going to copy the page.
Deleting pages
You can select or multi-select ([Shift]- or [Ctrl]-click) the pages you want to delete in the thumbnail view. Next, hit the DEL key or press the Delete button in the toolbar. You will be prompted for a confirmation before the pages are actually deleted.