Hyperspace was an alternate dimension that could only be reached by traveling at or faster than the speed of light. Hyperspace enabled starships to travel through hyperspace lanes across great distances, enabling travel and exploration throughout the galaxy.
When Earth was discovered by the Rakata their primitive use of hyperspace created a sort of "wall' in spacetime that warped the view of the Galaxy and beyond to those in the Earth system. In addition holonet transmissions were unable to be detected despite devices on Earth constantly looking, this was until the system was rediscovered by the Empire who dropped relay stations along the way to allow communications.