Ancestral health means living like people lived for hundreds of thousands of years before life got messed up - before food ingredients with names you cannot pronounce and high fructose corn syrup in everything, before fear of eating healthy fats and red meat, fear of being out in the sun, before we started exercising like rats on a wheel, before everyone was taking medicines that do more harm than good.
The ancestral health movement includes: Paleo, the Primal Blueprint, the Weston A. Price movement and other similar approaches. My primary orientation is Mark Sisson's Primal Blueprint, which is based on the Paleo Diet but is IMHO more sensible and easy to follow.
The premise of Paleo is that our ancestors evolved over millions of years to eat a certain diet. Agriculture has existed around 10,000 years, and much less than that in most of the world. That is not nearly long enough for us to have evolved to eat grains or highly processed foods such as seed oils. Most followers of Paleo no longer have a rigid "if cavemen didn't eat it neither should we" attitude. "Primitive" isn't always best.