Run allows you to share your site with people before you've paid, attached your domain, or gone live.
First, click the Publish button in the lower right of the CMS to capture all your latest changes. Next, click the View Live Site button in the bottom bar and your site will be published to a dummy domain which you can share with others. Keep in mind that while this dummy domain would be nearly impossible for someone to find, it's not password protected, so keep that in mind when you View Live Site.