Mason is the remains of a lunar crater that lies in the northeastern part. It is nearly attached to the eastern rim of the flooded crater Plana, and southeast of Bürg. Along the northern rim is the southern edge of the Lacus Mortis, a small lunar mare. To the south is the larger Lacus Somniorum. (wikipedia entry)
A crater is a circular depression likely created by an impact event. On the Moon they are named after deceased scientists, polar explorers, astronauts or cosmonauts.