The Monster world is the place where the Hormone Monsters and ohter monsters live and work.
The monster world is first shown in episode 10 of season 2 when Andrew Glouberman, Jessi Glaser and Nick Birch finds the portal in Nick Birch shelf after Tyler forgot to close it. They go through it and find out that it is the place where the monsters work and live. They go to the Appartment for puberty and to bring Nick a new Hormo Monster. There they find Gavin as a potenial candiate. They also meet Ricky and the others hormone monsters. But Maurice tells him that they cannot be here for to long because than they never get out of puberty. Andrew also meets the Shame Wizard in his office. Jessi Glaser sees the monsters of here, discusing about here future. There she also sees the Depression Kitty and decides to go with her to Depression Ward, a padded cell like room, instead of following Connie. The Kitty dont want to let her escape but at the end the others help here to get back in the normal world.