Just to give y'all a nice visual of what host is talking about here, check out this:
THis is a histogram of the overall scores of all 100 senators. The horizontal axis is the score (from conservative on the left to liberal on the right). The vertical axis is the number of senators with the score in that range.
It shows, in no uncertain terms, just how polarized our country is at the moment. There is one cluster of senators centered at around 15 and another around 95.
The 6 moderates host is referring to are the six blue bars in the middle of the graph, sitting there in the middle of the no mans land between the two sides.