Standard
The International System of Units is a standard that is agreed on by most countries. In it there is metre (or meter). It measures length.  A rod (made from a special material) is in Paris. The length of the rod said, years ago, how long a meter is. When people wanted to measure something, they compared it to the length of the rod. For practical purposes, the length of the rod was divided by 10 (to get dezimeter), by 10 (to get centimeter), and so on.