The document discusses the 90/10 principle, which states that 10% of life is made up of external events outside of our control, while 90% is determined by our reactions to what happens. It gives an example of how a spilled cup of coffee could lead to a bad day if one reacts negatively, but not if one remains calm and positive. The principle suggests we cannot control what happens but can control our responses, which dictates our experiences. Mastering the 90/10 principle involves not letting negative comments or unexpected events like traffic or delays ruin your mood or day.