I assume that the readers know the Bayes' rule already. If you are not familiar to it, read any kind of textbook about probability, data science, and machine learning. I recommend the book, which I learned Bayes' rule. Bayesians say that you cannot do inference without making assumptions. Thus, Bayesians also use probabilities to describe inferences. The author in the chapter 2 introduces some rules of probability theory and introduces more about assumptions in inference in the chapter 3.