What Does Zone Two Mean Physiologically?

A lot of people like hearing about the science, but then they also do want to have some sort of way to relate to it to take home. What does zone two mean physiologically? I think that's actually an interesting question. So between now and whenever I've thought about it for a year or two, you're going to see an evolution of thought. The practical take home definition would be a training intensity that does not accumulate much undue fatigue.