Chad Robertson's Oat Porridge Bread calls for "high extraction" flour. What is this? Thanks!

It's a way of milling flour. The grains are soaked, dried and then milled. They remove some, but not all of the bran and endosperm which means it has more fiber and flavor, but stores better because some of what can go rancid is removed. It's not so easy to find. You might try King Arthur or Bob's Red Mill or you could do a little research to see what the best commonly found flours you could substitute. Maybe a mixture of ww and bread flour?

His blog gives quite a seminar on flours if you want to go into greater depth.