Low-E stands for low emittance. Simply put, the glass is coated with a virtually clear material that acts to cut the transmission of ultraviolet rays from the outside to inside. Low-E glass improves the thermal efficiency of the window, cuts glare and prevents damage that ultra-violet rays can cause to interior furnishings. If you are in a climate that has long, hot summers, Low-E windows can make a Big difference in your air-conditioning bill, too.