Since its discovery nearly 10,000 years ago by Vedran physicist Rochinda, the slipstream has connected the galaxies together. Slipstream is an extension of our reality, an additional dimension that's integrally intertwined with our own. The slipstream is a place where quantum connections are visible as cords, especially the large and strong connections like those between huge concentrations of matter such as planets or suns. A spaceship that enters the slipstream can harness the energy of these cords and ride them from one star system to another.
One interesting thing about moving through the slipstream is that travel time between points has very little to do with the distance actually traveled. If a pilot is lucky, and the stream unfolds just right, the ship could transit between galaxies in minutes. But put an unlucky pilot at the helm and the same trip could take weeks or even months.
Luckily for the cause of interstellar commerce and communication, the more a certain path is frequently traveled, the faster, easier and more predictable the journey becomes. As a result, frequently-traveled routes between major Commonwealth worlds -- Vedra to San-Ska-Re, for example -- are safe and convenient.
Another unusual aspect of slipstream is the requirement of an organic pilot to guide a starship through the slipstream. At an intersection of pathways in slipstream space, both paths manifest the potentiality of being correct and incorrect. It's only when the pilot chooses a specific direction that this potentiality collapses and one path becomes right, and the other wrong. For reasons still not completely understood, organic beings tend to choose the correct paths, or more precisely, the very act of choosing makes the path they have chosen the correct one.
But strangely, computers -- even ones with artificial intelligence -- are incapable of this reality-altering guesswork. Even the most sophisticated starship in the Systems Commonwealth requires an organic sentient to pilot through the starlanes -- a prospect some sentients regard as deeply disturbing but others find comforting.