What is a painters undercoat?

Primer.

Sometimes also an underpainting in specific colors, usually earth tones. These colors are different (often contrasting) to the top-coat colors. The purpose of this underpainting is, to make the top-colors look more radiant. For flesh-color (portrait), painters in the old days used green. If you paint over that with light brown, it will look pinkish because of the green.