Description Editor is a tool of the Theme Tweaker add-on that aims at changing product and categories descriptions on the fly.
When enabled, the Description Editor tool will highlight where you can add new text or change the existing one. These are the product and category descriptions (brief and full).
You can edit the text of the description the very same way as you do it in the admin, i.e., you can choose the font family, size, and style, add images and video, lists, tables, and other things.
If you need to edit a text description on any other language installed in your store, switch the storefront to the target language, and translate the contents.
Don’t forget to save changes when complete.
You can find more info about translating a store to another language here.