https://docs.magento.com/m2/ce/user_guide/cms/editor-configure.html

Configuring the Editor

The WYSIWYG editor is enabled by default, and can be used to edit content on CMS pages and blocks, and in products and categories. From the configuration you can activate or deactivate the editor, and elect to use static — rather than dynamic — URLS for media content in product and category descriptions.


WYSIWYG Options

To configure the editor:

1. On the Admin sidebar, tap Stores. Then under Settings, choose Configuration.
2. In the panel on the left under General, choose Content Management.
3. Expand  the WYSIWYG Options section, and do the following:
a. Set Enable WYSIWYG Editor to your preference. The editor is enabled by default.
b. Set Static URLs for Media Content in WYSIWYG for Catalog to your preference for media contentthat is included in category and product field descriptions.
4. When complete, tap Save Config.