Publishing & domains
Publishing your site
Until you publish, your work is a private draft.
Preview vs published
The preview is a live server running your current code — real, but private to you. The published site is a separate copy served at your public address. Editing never changes what visitors see until you press Publish again.
That is why the Preview tab shows an amber banner while a site is unpublished: it is telling you that visitors cannot see any of this yet.
Publishing
Press Publish in the Preview tab. The first time, you choose your address — a free subdomain suggested from your project name. If it is taken, a suffix is added.
Re-publishing
Press Publish again after changes. The public site is replaced atomically; visitors do not see a half-updated site.