Plain English Meaning
Light bends when it passes through water, fuel, or other clear liquids. That bending can make what you see through the liquid look distorted, displaced, or hidden — which matters when you're inspecting an aircraft surface that has water on it.
Full Definition
The bending of light as it passes from one transparent material into another of different density, such as from air into water or from air into a layer of liquid on a surface. The change in density slows or speeds the light, causing its path to bend and shifting the apparent position, size, or shape of objects seen through that surface.
Context Anchor
During a visual preflight inspection, a pilot may notice light refraction when looking through fuel samples, water droplets, aircraft windows, or other clear surfaces.