Low-E stands for low-emissivity, a microscopically thin, spectrally selective coating applied to the glass surface in order to improves its thermal performance .  The Low-E coating acts as a thermal blanket, and manages the suns solar energy all year long.   In winter, as radiant heat trys to escape through the glass, it is reflected back inside the room.  In summer, the unwanted heat trying to enter through the glass is reflected back to the exterior.

In winter, Low-E Glass reflects more heat back inside, lowering your heating cost
In summer, Low-E Glass reflects more heat to the exterior, reducing your cooling cost and providing a comfortable room temperature
Some reasons that our doors are rated as energy-efficient doors: Reduced Conductivity with Warm-Edge Spacer Slower Convection and Conduction with Argon Gas Avoid Condensation and Decay Preventing Air Infiltration