Substitution is a concept where one thing is replaced with another. For example, in cooking, if you don't have butter, you might use margarine instead. This allows you to still create a dish without having all the original ingredients. Substitution can also happen in everyday life, like using public transportation instead of driving a car to save money. In mathematics, substitution involves replacing a variable with a specific value. For instance, if you have the equation x + 2 = 5 and you know that x is 3, you can substitute 3 into the equation. This helps solve problems and understand relationships between numbers.