The expression "f'(x) * g(x) + f(x) * g'(x)" represents the product rule in calculus. This rule is used to find the derivative of the product of two functions, f(x) and g(x). Here, f'(x) is the derivative of f(x), and g'(x) is the derivative of g(x). In simpler terms, when you want to differentiate a function that is the result of multiplying two other functions, you apply the product rule. This means you take the derivative of the first function and multiply it by the second function, then add the first function multiplied by the derivative of the second function.