km stands for kilometer, which is a unit of length equal to 1000 meters.
A meter is defined as the distance traveled by light, in a vacuum, in 1/299,792,458th of a second.
Accordingly, 1 km is the distance traveled by light, in a vacuum, in 1/299,792.548th of a second.
mm stands for millimeter and represents 1/1000th of a meter, or
1 mm = 0.001 meter
In other words, 1 meter consists of 1000 mm.
Both km and mm are units of length (or distance), in the SI system, and are extensively used for various applications. In several countries, the maximum speed limit for driving is given in terms of kilometers per hour or km/hr. Similarly, the speedometers of vehicles in several countries have readings in these units (km/hr).