To save an item for the future, you have a few options. You can click the Save button (looks like the end of a bookmark); that item is then added to your Saved folder. You can hit the Digg button (looks like a thumb-up); that item is then added to the Diggs folder. There are keyboard shortcuts for Save (s) and Digg (d).
You can also save items to Instapaper or Pocket, which are popular read-later mobile apps. If you go to Settings and add your log-in credentials for one of those apps, an icon will appear on the right-hand side menu for each item in Digg Reader. Simply click the share icon to save that item to the app you've chosen.