When you talk about "Low E with Argon" thermal windows, it is a question of a combination of a Low E film and replacing the air in the unit with Argon gas, an inert gas with low thermal conductivity.
The Low E is a thin layer of metal oxide applied to the glass while it is still hot so that these fuse together. Low E film allows short infrared (brightness) pass through, while it prevents long infrared (heat) from penetrating.
Argon is a gas with low thermal conductivity. The sealed unit is filled with this gas to replace the air to reduce energy losses, thus reducing the consumption of electricity for heating.
Low E thermal windows with Argon have excellent insulating properties and they exert effective heat control which significantly reduces heating costs. This combinations lets you enjoy a maximum of brightness throughout the year while keeping the house cool in summer and, conversely, warm in winter.