Probability is a measure of how likely something is to occur, expressed as a number between 0 and 1 or as a percentage between 0% and 100%. The higher the probability of an event, the more likely it is to happen. Impossible events have no chance of occurring, unlikely events have a small chance, likely events have a big chance, and certain events will always happen. The document provides examples of assigning probabilities to different events and discussing those probabilities with friends.