Section outlines add a Show sections switch to your header. When it's on, every section on the page gets a dashed outline and a label with its name, the same names you see in the theme editor.
- In Customize, open Theme settings → Section outlines.
- Tick Show the switch and pick an outline color.
- Click Save.
It's handy while you build your store or when you send screenshots to a developer. Turn it off before you launch if you don't want shoppers to see it.