The second moment of inertia is a geometric property that represents the resistance of a cross-sectional area to bending. It is calculated by integrating the square of the distance from each point in the cross section to a chosen axis over the entire area. The second moment of inertia is important in structural engineering because it determines the bending stiffness and strength of a structural member. A higher second moment of inertia indicates a greater resistance to bending and deflection.