The chain rule is a fundamental concept in calculus used to differentiate composite functions. It states that if you have a function that is made up of another function, you can find its derivative by multiplying the derivative of the outer function by the derivative of the inner function. This allows you to break down complex derivatives into simpler parts. For example, if you have a function like f(g(x)), where f is the outer function and g is the inner function, the chain rule tells you that the derivative is f'(g(x)) * g'(x). This technique is essential for solving many problems in calculus efficiently.