The second derivative test is a mathematical method used to classify critical points of a function, determining if they correspond to local minima, maxima, or saddle points. It relies on the sign of the second derivative: positive indicates a local minimum, negative indicates a local maximum, and zero means the test is inconclusive. The document also provides a practical example using the function f(x) = x³ - 6x² + 12x + 5 to illustrate the process of applying the test.