Double pane windows are made up of two panes of glass sealed with a transparent gas such as argon or krypton between them. Because the panes of glass are designed to fit precisely in the window frame, less gas is needed, providing a more energy-efficient window overall.
The two panes also provide an extra layer of protection and insulation for your home to keep your house warm in cold weather and cool in hot weather. This is why double pane windows are often rated to be more efficient and provide better insulation, they are often used in homes where the climates can change drastically.