Earthshine is a phenomenon that occurs when you can see the unlit part of the Moons disk. Normally it is difficult to see anything beyond the crescent, but sometimes light reflecting off of the Earth lights up the Moons surface enough that we can make out some details. More broadly, this is known as Planetshine.
It was not this bright in person, (looking more like the photo at the end of this post) but it makes for an interesting photographic opportunity if you just overexpose the lit part of the crescent. You can even see some stars in the photo. If the Moon was fully lit, you would not be able to take a picture capturing both surface details and stars in the frame.