Before anyone in the outside world can view your site, it must be published. Similarly, if you make changes to your site, no one can see them until you publish those changes.
Publishing a site also works as an editing place-saver. If you don't like changes you've made, you can discard those changes by reverting back to your most recent Published Version, or you can revert back to an another past Published Version by selecting Revert to previous version in the left editing panel.
This article provides more information on how to revert between Published Versions of your site, along with specific examples of how this feature functions.
How to Publish Your Site
To publish your site, click the Publish button in the top right:
When you hit Publish for the first time, you will be prompted to create a link. Watch this video for instructions on how to create and share links: