Subspace is a parallel dimension that allows superluminal travel. Its physics are poorly understood, but it is nonetheless used as the most common method of traveling the long distances between star systems.
The speed in which one travels through subspace is divided into tiers. The most common tier clocks at about 700,000 times faster than light, or about 45 seconds per light-year of distance. The highest tier, most often used by advanced civilizations and important entities, shortens that to about 4 seconds per light-year.