How do the prefixes in the metric system relate to the basic units?

The basic unit of length in the metric system is the meter. Grams are used to measure weight and liter is used to measure liquid capacity

besides, it is used so that you can express very large and very small numbers in a short unambiguous manner.

Example Instead of saying "three thousand million" meters

They classify the decimal place of the basic unit. ie, the meter is classified with milli (1/1000), centi (1/100), deci (1/10), Deca (10), Hecto (100), Kilo (1000).