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 preview domain which you can share with others.
Keep in mind that while this preview domain would be very difficult for someone to find, it's not password protected by default. To add password protection, go to your Site Settings page and in the Payments and Domain section, you can turn on password protection in the Preview Domain section.