I felt exactly the same way but I knew the explanation would be hidden inside some main thread that might have a poorly worded title and sure enough I was right.
The thread is called: “** Hub Preview Documentation **”
And really they should just write: “What is Hub? Documentation available here.”
But that’s what happens when you get people with different opinions of how to express something.
I would have chosen more specific and concise words.
HUB is described by Unity through this quote:
"The Unity Hub provides a convenient new way to manage your Unity projects and installations.
You can use the Hub to create your Project, associate a default version of the Unity Editor with
the project, and manage the installation of multiple versions of the Editor."
Anyway, enough of my Autistic babble, rather than just reply I’ve provided a link to the documentation.
Here you go: