Snell's Law describes how light bends when it passes from one medium to another, such as from air into water. This bending occurs due to a change in the speed of light in different materials, which is quantified by the law's formula: n1 * sin(θ1) = n2 * sin(θ2), where n represents the refractive indices and θ the angles of incidence and refraction. Understanding Snell's Law is crucial in various applications, including lens design and fiber optics. It helps explain phenomena like why a straw appears bent in a glass of water and is fundamental in the study of light and waves.