The Moon is in synchronous rotation with the Earth. This means that the side we observe is always facing us. That side has a darker region called the mare. The mare is more enriched in iron and is believed to be solidified lava pools. The Moon is constantly being impacted, forming many craters. When the crater forms, part of the surface melts because of the high velocity of the impact. This melt is called the impact melt flow because it flows over the rim of the crater onto the surface. The composition of the melt flow tells us about the composition of the Moon, since the surface of the Moon has been significantly altered through space weathering. Impacts craters excavate down a tenth of the crater diameter exposing the near subsurface
Eaton, Charlotte, "Spectral Properties of Lunar Impact Melt Flows" (2015). Aerospace, Physics, and Space Science Student Publications. 1.