The answer is: reduce the flow of heat passing through the window.
A double glazing window or insulating glass consists of two glass window panes separated by a spacer bar and seals to create a hermetically-sealed environment. Space between glass panes is filled under vacuum with either dehydrated air or an inert gas to improve insulation and prevent condensation within the unit. The physical properties of the dehydrated air used in the space between glass panes mean that it transmits less heat than normal air, this is because the molecules of dehydrated air are relatively inert and therefore have a lower thermal conductivity.