In short, the feature is a way to quickly access bookmarks: the Speed Dial page contains one to nine bookmarks, and it appears every time the user opens a new blank tab. The Speed Dial page shows dynamically updated thumbnails of each site, allowing users to quickly check for updates. In addition, users can access their favorite sites from anywhere by hitting CTRL and the number key corresponding to the Speed Dial bookmark number, or by just entering that number in the address bar.
Speed Dial aside, the new version of Opera introduces new developer tools for web designers as well as a few other miscellaneous changes and additions. A complete list of changes since Opera 9.10 can be viewed on this page. As for the browser itself, it's available from Opera's download page over here.