Good question, I keep seeing people mention it, but I have no idea what it is either

It was a thing from nostraisia, Japan Nostr does it is my understanding, where they don’t @ someone to reply. They just repost it, then post their reply. So you see them sequentially. I’m not sure why or what sort of benefit it is supposed to have. It developed as a convention very early in Nostr my guess is that it filled the need for quote reply before the devs had made that work? But this is just what I have picked up from context clues I might be a bit off.

Imagine if I just posted this comment but you had no idea what I was replying to. It was just free. A thought into the ether.