Function substitution is a mathematical technique used to replace a variable in a function with another expression or value. This process allows for simplification or evaluation of complex functions by substituting known values or simpler functions in place of the original variable. For example, if you have a function f(x) = x^2 and you want to evaluate it at x = 3, you can substitute 3 for x, resulting in f(3) = 3^2 = 9. This method is commonly used in algebra and calculus to solve equations and analyze functions more easily.