What is Layering?

I have a dear pal in HI who wants to learn more about art.  I talked to her about layering.  Whether with watercolors, colored pencils, or other art mediums, layering is integral to some art styles.  So I decided to show her the process.  Below are the 3 different layers I sent to her.  I still have more to go, as well as finishing that background!  Sorry the scanner picked up so much white space.  I didn't have time to crop it.  Ah, well.

Layer One:  Just the darkest darks & a hint of where lights & color will go:



Layer Two:  Adding more lights, darks and color

Layer Three:  More lights, darks, colors & some modeling...haven't started burnishing yet!