Hyperspace was an alternate dimension that could only be reached by traveling at or faster than the speed of light. Hyperdrives enabled starships to travel through hyperspace lanes across great distances, enabling travel and exploration throughout the galaxy.
At least during the High Republic Era, hyperspace was not truly understood by anyone in the galaxy. Hyperspace was a dimension of space-time alternative to that of realspace. It could only be accessed through molecular displacement, which was achieved by breaking the speed of light. It was coterminous with realspace, meaning that each point in realspace was associated with a unique point in hyperspace, and all adjacent points in realspace were adjacent in hyperspace as well. Additionally, every object in realspace (such as stars, planets, and asteroids) had its “shadow” counterpart in hyperspace. Aside from those well-established facts acknowledged by all competent astrophysicists and astrogation experts, other aspects of hyperspace admittedly remained a mystery.
The ability to travel through hyperspace occurred in nature. The space-based species known as the purrgil could naturally travel through hyperspace. At some point in the ancient past of the galaxy, an ancestral species unlocked the secrets of traveling through hyperspace and opened the galaxy up to exploration. Hyperspace travel was mastered as early as four millennia before the Cold War, as demonstrated by the existence of hyperspace sextants from that era. In the early days of the Galactic Republic, hyperspace travel between the Greater Seswenna and the Core Worlds required astrogating by hyperspace beacons, with numerous reversions to realspace necessary to ensure safe passage. Nevertheless, access to hyperspace travel proved to be vital to the Republic’s expansion.
Around two centuries before the Invasion of Naboo, starships across the galaxy were abruptly knocked out of hyperspace in the Great Hyperspace Disaster masterminded by the Nihil. Thanks to their path engines, the Nihil were able to preform acts with lightspeed thought to be impossible.
At the time of the First Order–Resistance conflict, the First Order discovered sub-hyperspace, which they utilized with their superweapon, Starkiller Base. The firing of Starkiller Base during the Hosnian Cataclysm in 34 ABY released enough energy to create a temporary rip in sub-hyperspace. This made the disaster visible from across the galaxy instantaneously, for a short time.