Probability is a branch of mathematics that deals with the likelihood of events occurring. It quantifies uncertainty by assigning a number between 0 and 1 to an event, where 0 indicates impossibility and 1 indicates certainty. The probability of an event can be calculated using various methods, including experiments, historical data, and mathematical formulas. It is widely used in fields such as statistics, finance, science, and engineering to make informed decisions based on uncertain outcomes.