Interdimensional Portal

The  is some kind of machine hidden away in Stan Pines' secret underground laboratory.

History
The key to starting the machine is a complex algorithm that can only be completed by owning the journals 1, 2, and 3. Stan Pines has searched for 2 and 3 for an undetermined amount of time.

After Dipper and Mabel Pines defeat Gideon Gleeful's giant robot, and Stan reveals Gideon for the fraud he is, Stan steals journal 2 from the wreckage. Later, Dipper shares that he owns book 3 and Stan takes it on the pretense that fantasy books such as those were making his imagination run wild.

Stan takes the two books and combines it with journal 1 and inputs the data gathered from it into a large machine. This activates the portal.

Function
From what Dipper gathered from his section of the algorithm in book 3, it is some kind of strange, futuristic, super weapon. However, the cryptogram written on the page describes it as a portal that opens a gateway to infinite new worlds.