Each box is a poll that has been corrected. At the bottom is a "Keep reading" link which opens up the details. From that page you can see:
Interspersed are summary map boxes. The "Keep reading" opens up a summary page that shows each battleground state and the links to the recent adjusted polls for that state.
What I like is that he has some polls where the adjustments are negligible, where some show a small Biden increase, but most show a swing from something like a Trump-3 loss to a Trump+2 win. Then there are the corresponding Senate races with the same adjustments.
We won't know how accurate it is until after November 3, but he suggested that Trump wasn't losing by 10 in 2016...
-PJ