D'Arrest is a lunar crater that is located in the lava-flooded region to the west of the Mare Tranquillitatis. It lies to the southeast of the crater Agrippa and northwest of Delambre. Just to the northeast are the small, bowl-shaped craters De Morgan and Cayley. The outer rim is broken in several places, with a gash at the south end and a wide gap to the northeast where only a low ridge remains in place of the original wall. (wikipedia entry)
A crater is a circular depression likely created by an impact event. On Phobos they are named after deceased scientists involved with the discovery, dynamics, or properties of the martian satellites, and people and places from Jonathan Swift's "Gulliver's Travels".
Image Source. Marker locations are approximate and may not always appear to align with image on map.