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 the area where you can add new text or change the existing one.
These are the products and category descriptions (both brief and full ones).
You can edit the text of 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 store frontend to the target language, and translate the description.
Don’t forget to save changes when complete.
More info about translating a store to another language, you can find here.