Windowing is an optical phenomenon that occurs when light passes through a gemstone without being refracted. If windowing is present in a gemstone the observer can look through the stone at certain angles as if it were a window or can observe large dark areas in the stone (caused by light leaking out the window).
A well cut stone should reflect so much light that nothing is seen behind it. However this statement presumes that you are looking straight down through the top of the stone, and the stone is not tilted at all. Tilting the stone even slightly will change the angles of light hitting the pavilion facets. Each type of gem has its own range of "tilt brilliance". The lower the refractive index the less tilting it takes to window the stone.