"I have guided a Heighliner through foldspace, brother. I hold the galaxy in my mind, I see mathematics."
- D'murr Pilru, Dune: House Atreides
Foldspace is a term that comes from the universe of Dune, developed by Frank Herbert. It is the region of space through which space travel can be conducted at effectively greater than light speed. The concept is similar to that of hyperspace.
In order to safely travel through foldspace, prescience is required to select the best path. Members of the Spacing Guild (called Navigators) provide that service with the aid of the spice melange. To best utilize that service, Navigators pilot huge ships called Heighliners that can be loaded with vast amounts of cargo, smaller ships, and passengers.
Traditionally, direct communication through foldspace is considered impossible. However, C'tair Pilru developed a device while trapped on Ix called a rogo transmitter to communicate directly to his twin brother D'murr. The procedure was duplicated, but it requires two minds that are already closely linked (like that of twins), and so was too expensive to be developed for general use.
Note: some information in this writeup contains information from the prequel series written by Brian Herbert and is not necessarily considered part of Dune canon.