When creating your website you will need to save your changes as you progress. The 'Save Draft' button is located at the top right of the edit (see screenshot below).
Whenever there have been changes from the last save the 'Save Draft' button will be highlighted.
The saved changes will only be a draft and not made public until you 'Publish' the changes. The 'Publish' button appears just to the right of the 'Save' button. Simply click this to publish the last saved version of your site.
Once you have published your site for the first time, it will be live to view on your subdomain.
By having the option to 'Save' and then 'Publish' it allows you to save draft versions of your site and launch it once you are happy with all the changes.