If your website is on the Business or Online Store plans on Avoori, you can access “Add Post Editor to Taxonomy Description Textarea” setting by going to your website admin > SEO > Advanced.
Taxonomy in WordPress is the way site owners tag posts together. For example, Categories and Tags are two popular ways people group posts together in WordPress. So these are taxonomies.
Checking the following option would add TinyMCE (a popular web editor) to the term description text-area.