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 internal reactions to what happens. How we react to situations, both positive and negative, has a significant impact on our experiences and outcomes. The document provides an example of how having a bad reaction to a spilled cup of coffee can negatively impact one's whole day, while responding calmly would lead to a better outcome. It encourages readers to apply this principle by not letting external events ruin their day through overreacting, but instead focusing on controlling their internal reactions.