Substitution is a concept where one thing is replaced with another. For example, in mathematics, if you have an equation like x + 2 = 5, you can substitute x with a number, such as 3, to see if it makes the equation true. This helps in solving problems by simplifying complex expressions. In everyday life, substitution can be seen in cooking. If a recipe calls for butter but you don't have any, you might use margarine instead. This allows you to still create the dish without having all the exact ingredients, showing how flexible and practical substitution can be.