Once you have tested your application and are ready to go public, you can add a domain and connect a specific version to it. In future you can continue to create new versions, but only one version is connected to your domain, allowing you to test each version thoroughly before it goes public.
Add a Domain
To add a domain just open the domain manager, then enter the domain you want to add. You will also need to go to your hosting / DNS system and add a record to direct your domain to the vercel hosting system.
Choose a version
Each time you create a version, unless you chose to publish it directly to the domain it will only be visible at the preview url.