Probability is a branch of mathematics that deals with quantitatively measuring the likelihood of an event or experiment having a particular outcome. An event refers to one or more outcomes of an experiment. The probability of an event is calculated by taking the number of outcomes that make the event occur, and dividing it by the total number of possible outcomes. For example, the probability of getting 3 red squares when drawing randomly from 9 squares is 3/9, and the probability of getting 2 yellow squares is 2/9.