Phlegm is the thick substance in your throat when you have a cold or wake up in the morning with a congested throat. You get that substance out by coughing or clearing your throat, and that substance is phlegm. Sounds awful to think about but really is no big deal. Some people use the throat clearing to draw attention to themselves so that they can speak to a crowd which isn't paying attention. Phlegm at one time in history also was thought to be one of the four humors which caused apathy or calmness. In this book, phlegm is another side topic which is discussed almost as a diversion.