Okay, I was shocked there for a moment. Actually it's very easy. I know, you probably aren't interested in this, but it is really easy.
Let's take a meter as an example. A meter is just a measure, like a foot or a mile. Of course it is scientifically defined, but this doesn't really interest us non-physicists. We have a meter and that's it, we don't have anything else to measure length.
Of course, it would be uncomfortable to always have to write: "I traveled 100 000 meters" or "A bacterium is 0.000001 meter" and therefore we take the decimal power and give it a funny name. Instead of 100 000 meters we write 100 * 10^3 meters. And we define 10^3 as kilo, short k. That means 100 000 meters become 100 kilometers, hence 100 km. The same for small numbers. 0.000001 becomes 1 * 10^(-6), 10^(-6) is called micro, hence a bacterium is usually around 1 micrometer, 1 μm in size.
No you just have to remember the name for the decimal powers in an area you need it, for normal people it's millimeter, centimeter, kilometer. And then you do the same for kg, for mol, for K, and so on.
Is that clear? Sometimes I think about becoming a teacher after my PhD ;-)