The first thing you will notice is that every page of your issue is listed in this panel. If you have a particularly long publication with a lot of interactive features, you may find that your panel has a paging option at the bottom.
Headings with triangular indicators are clickable as there are interactive modules on the page. Pages with interactivity can be expanded and collapsed to show and hide different interactive modules.
In the interactivity for your issue, make sure you name the pages and modules. This can be done within Page or Module Properties. This will make the Page/Module column in the table clearer to read.