Questions regarding to risk stratification

As I finished reading the book Risk Stratification - A practical guide for clinicians, I get so confused by the "goal" of risk stratification. I thought it would be easy for me and I more or less understand, but the clinical concept is so confusing!

I will just take notes of some questions here. Hopefully, I will understand them later.

1. In Page 112, it says that only when the epidemiology problem is worked out, we should consider risk stratification. Then, what is the population-based epidemiology? What does it try to solve?

Okay, I found a dataset that I might be able to use for practice: http://archive.ics.uci.edu/ml/datasets/Thoracic+Surgery+Data