The Mother is a female-shaped mass of flesh pinned to the floor. She acts as a portal to bring small creatures into the nightmare world.
Mothers are stationary monsters that will continuously spawn Tremers until they are defeated. The Mothers themselves have no means of attack other than through the monsters they bear.
The Mother acts as a conduit of introducing foes into the Otherworld, such as a mother giving birth to children in the real world.
It is possible that the monster is a representation of life, as the protagonist's actions affect many of the other character's lives in Book of Memories. It could also represent the barrage of obstacles that spring up in the protagonist's life. Instead of dealing with each one individually by manipulating the past (the Tremers), it is possible that destroying the Mother may embody the character adopting a more universal perspective on the obstacles.