A Transportalizer is a pedestal-shaped device used to transport people and objects to other places. The central stairwell of Jade's house features a series of Transportalizers (Jade almost never takes the stairs). There is also one located in the Skaianet Laboratory near Rose's house, which, in Act 3, Jaspers's body is accidentally teleported by.
The post-scratch kids' Frog Temple contains two transportalizers, one yellow and one purple. They presumably allow travel to Prospit and Derse, which would explain how Grandma was able to steal the fourth wall from Derse and bring it to Earth (and, if the transportalizers are in all incarnations of the Temple, would explain how Grandpa did the same). Another transportalizer was found on LOCAH, which was also shown to lead to Derse.