In a nutshell, it's the shell: Everything that separates and protects indoors from out-including exterior walls, insulation, roofing, foundation and flooring, windows, and doors.
This was your grandmother's approach to air-conditioning: air that flows from one side of a room or building to another, between open windows or vents. It's a natural, passive cooling method. And there's no installation to worry about; it's a total breeze.