This is just a shortcut to Chrome bookmarks page.
It's just an alternative of the "Bookmarks" menu item accessible from the Chrome apps list and the Chrome App Launcher. Click on the "Bookmarks" icon and you'll open the Bookmarks page of the browser. It does just that and nothing more...
Get it from Chrome Web Store »
Some users seems to be confused why the shortcut isn't added to the toolbar. Well, apps (like this one) can't do that. But extensions can. To add the shortcut to the toolbar you just need to install the extension version.
Long story short: