What is all this talk about "joining a polycule"?

I see posts about this on here daily. I've been practicing poly/enm for well over a decade, mostly in New England. Here at least, people enter into relationships with individual people, not an amorphous blob.

What in the world do people mean by this? Do your polycules have a Google Form application or something open to the world? Is everyone obligated to date everyone in it? Is this something regional?