When the moon is a thin crescent, sometimes you can dimly see the rest of the moon. This is because the moon is illuminated by light reflecting off of the earth. If you were living on the Moon, you would see the earth go through phases, waxing and waning, like the moon does when viewed from earth. In fact, the phases would be inversely correlated, when the earth is roughly between the moon and the sun, we see a fully illuminated moon, but the earth would appear all shadowed to a lunar observer. When the moon is between the earth and the sun, we see a mostly shadowed moon, but the earth would be fully illuminated if you were looking at it from the moon. You would be able to walk in the dead of night and see where you were going, because the full earth in the lunar sky would cast bright earth beams, that is, lots of earthshine.
As an environmental philosopher, I like to use questions from ecology to test ideas of ethics, theology and what it means to be human. So the idea of being illuminated by light reflecting off the earth is a metaphor to good to pass up.
And as someone living in Appalachia, I like the idea of earthshine in another way as well.