Total internal reflection occurs when a light wave traveling in a denser medium, like water or glass, hits a boundary with a less dense medium, such as air, at a steep angle. If the angle of incidence exceeds a specific threshold, known as the critical angle, the light is completely reflected back into the denser medium instead of passing through. This phenomenon is essential in various applications, including fiber optics, where it allows light to travel long distances with minimal loss. Devices like optical fibers utilize total internal reflection to transmit data efficiently, making modern communication systems possible.