If you click on Hubs at the top of the page you will see they come in catergories: Hot, Best, Latest.
Once you are out of the latest catergory ... Hot and Best are either getting a lot of traffic and views or have a high Hubscore. That is what I think it means, so you need to be in the 90's at least for Best and Hot ... getting a lot of traffic/views.
Not sure that this is the best explanation but I hope it helps until a wiser and more experienced person on HubPages can give you a better definition of it.