A dimensional gate, artificially constructed, that allow ships to travel along an infinite line much quicker (100x or more times speed of light) than regular engines can permit. You need a gate at the start and end points, otherwise you will become stranded.
It provides a stable 'function' in local space to allow slowspace (normal) to touch the mirror horizon. A perfect definition of a 7 dimensional math formula, expressed in 3 dimensions.
Mirror horizon causes madness to anyone exposed to it directly due to simultaneously experiencing all possible outcomes of your current decisions, so the gate actually provides a safe transition point where the crossover is more or less normal.
Because it's a linear gate, as noted in my original reply, navigation is one way. The only group that actually can navigate the mirrorspace freely is the pirate group 'The Laughing Swarm', due to some alien relic they found. Everyone else uses gates or Alcubierre drive based tech.