Prefixes are put on the front of a unit to denote what power of 10 is being represented. For example, in "kilometer" the prefix "kilo-" is attached to show that the unit is now 10^3 times greater than the base unit, in this case the meter.
The same prefixes are used for all units. For example, a kilogram equals 1,000 grams, just like a kilometer equals 1,000 meters.
The official prefixes are International Bureau of Weights and Measures, but there are national differences. For example, the official prefix for 10^1 is "deca-", but in the U.S., the spelling "deka-" is preferred.
The metric prefixes are taken from Greek and Latin.
By using the prefixes, the metric system has consistent conversions between all units, unlike the imperial system, which has varying conversions, such as 16 ounces to a pound and 2,000 pounds to a ton.