The meter is a basic unit of length, there are prefixes that help to bring the measurement close to the desired effect. Deca x10; hecto x100; kilo x1000; etc... The kilometer is used for cars driving distance and a runner's distance. When you need to make things smaller: deci -10; centi -100; milli -1000. When you measure the width of the iris you may use the millimeter. The meter was a stick stored in France. It was the agreed length used for scientific measurement. Later a more accurate measure was how far a laser traveled over a certain amount of time.
