The 90/10 principle 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. How we react to situations, both positive and negative, largely dictates our experiences. The document provides an example of how reacting negatively to a spilled cup of coffee can ruin one's whole day, while responding positively can prevent negative consequences. It encourages applying the principle by not letting external factors like traffic or delays affect one's mood and instead focusing on how one can respond constructively. Mastering this principle can significantly reduce unnecessary stress and problems in life.