The metric system is a system of measurement used throughout the world and within the scientific community. When measuring with the metric system, prefixes are used to describe the quantity of a unit. For example, the metric unit for distance is the meter. In order to describe a distance of one thousand meters the prefix kilo can be used. One thousand meters is equal to one kilometer.
In the metric system a group of base units is modified by prefixes in order to display larger or smaller quantities. Different units describe different forms of measurement.
If a planet is 5,000,000,000 meters away, you can say that the planet is 5 Gigameters away. Giga is a prefix that multiplies the base unit (meters) by 1,000,000,000 times.