Dead Worlds are mentions of the afterlife as a place one arrives to after death. According to storyboard artist Jesse Moynihan, the reason why there are 50 "dead worlds" will be explained later on in the series. The first mention of a dead world was in "The New Frontier" by Jake who says one must transcend into Glob World.
The 9th Dead World is only a mention in the episode "James Baxter the Horse". When Jake and Finn were attacked by an angry ghost the situation was dire, resulting in Jake to think about their coming death. He says that since a lot of dead people will be there they might become separated so as a way of identification Jake tells him to listen for a rhythmic clapping sound that will be performed by him.
The 37th Dead World is a plane where souls go when they die, similar to the 50th Dead World that Ghost Princess and Clarence ascended to. Jake's spirit traveled to the 37th Dead World when he was killed by the Wand of Disbursement on Mars. Death appears to inhabit this world and Abraham Lincoln is able to visit it at will. The 37th Dead World may be related to Land of the Dead, as Death is shown to dwell there in "Death in Bloom."
The 50th Dead World is a location, theoretically similar to a heaven, mentioned several times in "Ghost Princess." Nothing is known about it, except that ghosts ascend there when they learn the cause of their deaths. Known ascendants are Ghost Princess and her love, Clarence.