If you stand outside in sunlight, you are exposed to three different types of light: infrared light, visible light and ultraviolet light. When you are inside looking out through a window, you really only need to see the visible light in order for the window to do its job. The window can block the ultraviolet and the infrared light and you will still see the same thing.
The advantage of blocking the ultraviolet light is that it prevents color fading on rugs and furniture. The advantage of blocking infrared light is that it keeps summer heat outside the house and it keeps winter heat inside the house. In other words, it improves energy efficiency of the window.
This article demonstrates the huge difference low-e glass can make to winter indoor comfort:
Imagine a cold night with an outside temperature of 0 degrees and a 15 mph wind. The inside temperature of a single pane window would be approximately 26 degrees. Regular double pane glass might register 35 degrees. Hard coat low E glass would be very near 49 degrees. And weighing in at champ would be soft coat low E glass at 62 degrees.
Imagine standing next to a window with the glass temperature at 62 degrees F. Now imagine standing next to the same window with the glass temperature at 26 degrees F. The 26 degree glass will be able to form frost on the inside of the glass. The 62 degree glass has nearly the same temperature as the indoor room air. Low-e glass means big energy savings.
Low-e glass has either a tin (known as hard) or silver (known as soft) coating on the glass to block infrared light. It pays to know the difference. This is the same kind of technology used to make one-way mirrors, but not so thick.
The metal coating, however, does block some visible light. A low-e window might block half of the visible light hitting the glass. This may have an effect on houseplants, as seen here:
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