Admins (I am not an admin here) can edit a special “about” topic within each category. That controls the category’s little blurb that appears in places.
Discourse, I believe, does not include any sort of “excerpt” with each topic in a list. I’ve looked all through the admin settings (on one of my Discourse communities where I am an admin.) There’s even a plugin that does it: Topic List Previews , so I’m pretty sure it’s simply how Discourse has decided to design the software.
If you want to dig (bearing in mind that we couldn’t change this behavior, one of the admins here would have to) I’d recommend heading to https://meta.discourse.org/