Rendering is a process of computing and displaying a scene. The computations are based on pre-defined algorithms that take into account: the geometry of the objects (modeling), the materials applied to those objects (metal, marble, wood, etc.), the lighting, the point of view and, finally, the interaction that all those elements may have between them (shadows, reflections, etc.).
If no material is assigned to an object, Amapi 3D will use a default color. Default lighting values will be used as well, if none were specified.