During the winter, heat (long wave radiation) in the room naturally wants to escape through the glass, but low-E glass reflects it back into the room keeping it warm. If you touch a standard pane of glass in the winter it feels very cold, even if the room itself is hot. If you touch the inner pane of Low-E glass, it feels much warmer. During the summer months it is hot outside, but the low-E glass does not let the infrared thermal radiation (long wave) pass through it reducing the overall solar heat gain of the building.
Summer mode. Light is admitted through the glass, but heat from outside is reflected, helping to reduce cooling costs.
Winter mode. Light is admitted through the glass, but heat from interior spaces is reflected back into the house interior by the low-e coating.