Sometimes you need to get the original copy of a theme file back. With all files (except ones in the assets/ folder), Shopify gives you the ability to restore the original file that came with the theme.
This is handy if you've made some edits to a file and need to revert them to a previous file version. Sometimes it's an app that injects code into a file and breaks something.
How do you know what files have been edited in your theme? You can notice which theme files are edited with a small purple dot:
If you don't see any dots, it's likely because you duplicated your theme so it thinks it is brand new.
Shopify has a built-in feature for this which you can find instructions for here: