The Concept of Normal
A normal in 3D world refers to the vector that is perpendicular to a surface. A face or polygon contains only one normal direction. The normal indicate the 3D programs the front and back sides of faces. The front faces (polygons with normal) will be rendered and then appear in the 3D programs while the back faces do not. Therefore, if you turn the camera to the back faces, they look transparent and you can see everything beyond.
The normal directions (blue lines)