A window is an opening in a wall that you can see through. Most windows are fitted with glass and can be opened to let air flow through them on a warm spring day.
Buildings have windows so that the sun can brighten up indoor spaces during the day. You can also use window to mean just the glass pane inside the frame: "Sorry that my baseball broke the window!" Cars have windows too, which can be rolled up and down, and department stores use their large windows to display items for sale. And on a computer, a window is an area containing information that's enclosed by a window-like "frame."