Excellent question. Topics and Feeds, to my knowledge work like this:

Topic : can only be created and edited by admins (maybe moderators, too?). Designed to help new people find communities to subscribe to without being a free-for-all. For example when I created the “Wetshaving” Topic, I didn’t include one or two wetshaving themed communities found elsewhere on the Fediverse because those places are barren, so they aren’t worth following in my opinion.

Feed: A feed is created by, used by, and only visible to a particular user. You can use a Topic as a template of sorts… so if you want to follow the same shaving communities that I’ve curated in the wetshaving Topic, you’d visit that topic (Explore -> Topics -> Wetshaving) and click Create feed.

I had a brief conversation with the PieFed developers today because I was trying to wrap my head around it, and it’s a bit buggy and convoluted at the moment. But hey, it works.