The document discusses the Fundamental Theorem of Calculus. It states that if f(x) is an integrable function and g(x) is an antiderivative of f(x), then the integral of f(x) from a to b is equal to g(b) - g(a). This theorem links the concepts of differentiation and integration by showing that one can find the area under a curve by subtracting the antiderivative evaluated at the bounds. It is an important foundational theorem in calculus.