What is moment of inertia and its significance in beam design?

Moment of inertia (second moment of area) is a geometric property that indicates how a cross-section's area is distributed about an axis. Higher moment of inertia means greater resistance to bending and less deflection under load. I-beams are efficient because material is concentrated far from the neutral axis, maximizing moment of inertia. It appears in flexure formula (stress = My/I) and deflection calculations.