"Reflectivity" is a fancy way of saying how much sunlight a satellite will reflect.

A satellite with a 0 percent reflectivity will absorb all light and reflect none. Black holes reflect 0 percent of all light encountering it because 100 percent of that light is being absorbed.

An object with a 100 percent reflectivity reflects all of the light and absorbs none. There is no object on Earth that reflects 100 percent of the light shone on it, however a well polished mirror comes close at 99 percent.

All objects in the universe, with the exception of black holes, reflect light in some way. How they reflect light is dependent on the colour of the incident light and the colour of the object. For example, a red object will easily reflect red light and absorb green light. A black object will absorb much of the light (of any colour). A white object will reflect most of the light (of any colour).

Satellites are mostly metallic in their structure, so many are very reflective. However, some are coated with coloured or black paint to impede visual detection. Highly reflective satellites with low ranges are easily viewed by satellite observers here on Earth. The high gain antennae of the Iridium satellite can reflect high percentages of the sun's light, giving us brief yet spectacular light shows. Through Iridium, humanity has created the third brightest celestial object in the heavens, next to the Sun and Moon, outshining the planet Venus by up to 100 times!

Since satellites are very complex pieces of engineering, they contain many different components, each with its own specific reflectivity. The apparent reflected sunlight contains the culmination of all the reflected sunlight from all the satellite's individual components.

Example: Two identically sized pieces of charcoal are used in a reflectivity experiment. One is left alone, while the other is wrapped in aluminum foil. It is obvious that the aluminum foil will reflect more light than the plain black charcoal. When viewed from a distance with the same range, size, phase and orientation, the aluminum foil greatly outshines the plain black charcoal by many orders of magnitude.

If two satellites have the same size, range, phase and orientation with respect to the observer, the more reflective satellite would appear brighter than the less reflective satellite.

The two pieces of charcoal are placed side by side.

From a distance, the aluminium foil greatly outshines the black charcoal.