In Traditional Chinese Medicine (TCM), phlegm is considered a pathogenic factor that can significantly impact the body's physical wellness and mental health balance. Phlegm is viewed as more than just the mucus produced by the respiratory system. In TCM, it's a broader concept encompassing various conditions marked by the accumulation of fluids in the body. Excessive phlegm can obstruct the flow of Qi (vital energy) and disrupt the harmonious functioning of organs, leading to imbalances and health issues. 

In the most basic of terms- it gunks up the pipes and makes many bodily systems sluggish.