Let's say that you have published a site and you have shared it on the Internet. The site will be live and can be accessed by the public. We say that the site is in a published state.
You can schedule a publishing time while publishing the site by clicking on Publish later and choosing the desired date and time.
Now you need to make some changes to the site. As you are making changes to the site, you don't want all these changes to be live to be seen by others. That's because you are still working on the site. We say that the site is in a draft state.
When a site is in a "draft" state, then others can't see the changes made to the site. They will see the site as it was "published". Once you are happy with all your changes in the "draft" state then you can click on "Publish" to publish the changes made to the site. Now others can see the changes made to the site.