Underpainting is an initial layer of paint, which serves as a base for subsequent layers of paint.  It is usually done in monochromatic shades of brown or gray, but not always.  It serves a number of purposes:
-  work out contrast and values using a single color       (without having to worry about color matching or mixing complex colors)
-  work out composition
-  adds a base or an initial layer of paint
-  don't worry about small details...just the big values..
- increases the luminosity and depth of color and creates an inner glow
- blank canvases can be intimidating and underpainting takes away that vast whiteness