The expression "f'(g(x))" represents the derivative of the function f evaluated at the point g(x). In calculus, the derivative measures how a function changes as its input changes. Here, g(x) is another function that transforms the input before it is used in f. This concept is part of the chain rule, which is a fundamental principle in calculus. The chain rule states that to find the derivative of a composite function, you multiply the derivative of the outer function f by the derivative of the inner function g. Thus, "f'(g(x))" helps in understanding how changes in g affect f.