Include short blurb on individual tag's (term?) listing page

Very much a noob here. I’m trying to set up a page using the Index theme.

I think what I’m asking about is called a taxonomy term page (and is a list kind of page?), but in case I’m wrong on the terminology: Taking Index’s example site, say you click on the tag css. This takes you to a page that simply lists all posts that have that tag. Makes sense.

What I’d like to do is include a short blurb (eg “CSS stands for Cascading Style Sheet, and it’s used…”) on that page, between the “Index/css” header and the “Title | Tags” listing.

I would strongly prefer for the blurb to be optional, so if some tags don’t have one they’ll just come out the way they currently do, not generate errors.

If there’s a choice between the blurb being a Markdown file vs a metadata/param/variable sort of thing, I’d prefer Markdown, but that’s not a hard requirement.

My best guess is that this means I’m looking for a taxonomy list template? But “Taxonomy list page templates are lists and therefore have all the variables and methods available to list pages” isn’t enough detail to get me any further ahead at my current level of knowledge.

I’m guessing there’s a .md file I need to create… somewhere? And then an existing template file… somewhere (which is already generating the existing tag-listing pages), into which I need to insert… an instruction of some kind… to pull in the corresponding Markdown block (if there is one for this tag, else just carry on without it)…?

Thanks for any pointers.