It stands for low-emissivity. It’s the condition that allows different types of glass emit low levels of radiant thermal energy. The short answer is that it reflects and bounces back heat signatures. In the case of windows, it would be the sun. If you’re looking to save on energy bills and you won’t want your home really hot in the summer, get windows with Low-E so that the sun’s ultraviolet rays bounces back – away from your home to keep it nice and cool during the summer.