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

Prefixes are used to modify the basic unit

Explanation:

The prefixes of any unit of the metric system signify a unit either larger or smaller than the base unit. It can be said that the prefixes are used as modification of the base unit

For example  

Second is the base unit of time (as per the MKS system)

If we add prefix “mili” the unit of time will become millisecond and this signifies a unit which is smaller than second as

1 second = 1000 milisecond

Like wise there are other prefixes such as micro, nano etc.