A fork is a branch of your site's content. Same site, same address, same hosting — but the pages you are working on are set aside until you decide they are ready.
What you need first
Forks are built on database sync, so the site needs its database backed up to a repository. Site Settings will offer to set that up if it is not done yet.
Create a fork
In Site Settings, create a fork and give it a name — "redesign", "spring-copy", whatever you will recognise later. PRSS switches to it immediately, and everything you edit from then on happens on the fork.
Work on it
Edit, preview and rebuild as normal. What you publish still comes from the official branch, so readers see the old site until you say otherwise.
Promote it
Happy with it? Promote the fork. Its content is committed onto the official branch as a new commit, so every device agrees on what is official and nothing is rewritten behind anyone's back.
Change your mind
Reverting a promotion puts the previous content back — also as a new commit forward, never by erasing history. Reverting a revert is a redo.