Fubini's theorem is a fundamental result in calculus that allows us to evaluate double integrals. It states that if a function is continuous over a rectangular region, we can compute the double integral by integrating one variable at a time. This means we can first integrate with respect to one variable, and then integrate the result with respect to the other variable. The theorem simplifies the process of finding the area under a surface in three-dimensional space. It is particularly useful in applications involving multivariable calculus and helps in solving problems in physics, engineering, and probability.