Focus Mode clears everything off the screen except your page, so you can write without distractions. Every editor feature is still available, and your work keeps saving as you type.
Turning Focus Mode on and off
In the Manuscript Editor, click the Focus Mode button in the toolbar.

To leave, press Esc, or move your mouse to the top of the screen and click Focus Mode again.
What changes in Focus Mode

The browser goes full screen.
The sidebar and the editor header are hidden, so you'll only see your page.
The page text is slightly larger, with more space between lines.
The Notes Panel and the ruler are closed. They come back the way you left them when you exit.
The toolbar slides out of view. A Hover to view formatting options tab marks where it is.
Using the toolbar in Focus Mode
Move your mouse to the top edge of the screen and the toolbar slides down, with every control available, including the Notes Panel button if you want your notes back while you write.

What Focus Mode doesn't do
It never changes or removes your writing.
It doesn't affect manuscript or worldbuilding versions.
It doesn't turn off any editor features. Everything is one hover away.
Many writers draft in Focus Mode and switch back to the full view when revising, when they want the sidebar, outline and feedback in sight.