It is a giant wall surrounding the entire Keystone Quadrant meant to act as a force-field fence to enclose the mentally deranged that once inhabited Halfworld
The Galacian Wall was supposed to be a vast impregnable force-shield prohibiting anyone or anything from entering or leaving the Keystone Quadrant
Incredible Hulk #271 (1982)
the Hulk mysteriously penetrated the Galacian Wall into Halfworld (Incredible Hulk #271, 1982); the Galacian Wall was shut down so Rocket Raccoon and his allies could leave Halfworld (Rocket Raccoon #4, 1985)
The Galacian Wall was built by an ancient civilization of humans in order to protect the unfortunates of their society. The mentally challenged Loonies were put on Halfworld to live out their days in peace and harmony along with the robots and animals that shared the world with them. The humans that put them there meant to keep them safe by allowing no one into their new home as well as shutting the Loonies in, unable to travel to other worlds. Even though the Galacian Wall was supposed to be an impenetrable force-field, only one being has managed to break the barrier. That individual was the incredible Hulk. After the devastating Toy War, the robots which shared Halfworld with the Loonies, found a way to turn off the force-field, so they may leave the planet along with some of the intelligent animal life like Rocket Raccoon.