What is the function of a double glazing window and function of each part?

The reason for using a double glazed window is that the two window panes trap a layer of air between them, which provides much better thermal insulation than a single pane of glass would provide, so a lot less heat is lost through the closed window during cold times of the year. During warmer times of the year, if you have air conditioning, then less heat enters the house through the closed windows.