The advantage and key to the S.I. system of units is its simplicity.
Any measured property has a unit that is a combination of some of the seven base units.
Larger and smaller multiples of these units are defined by mutliplying by powers of ten. The names of these units are derived from the base unit through the use of prefixes.
An example from everyday life illustrates this: We are all familiar with the kilogram. Effectively, this derived from the gram. The prefix "kilo-" signals that it is 1000 times as large as the smaller unit: i.e. one kilogram = 1000 grams.
(However, having said that, we will see in the page on base units that the base unit here is actually the kilogram, rather than the gram. Science it not always entirely logical!)