The nexus is the heart and soul of the Rift Network. Helios can create the most rifts to most stars in the galaxy and is protected by causing the rifts to form to it to be unstable unless you have a key which will cause the nexus to stabilize the incoming rift. This structure is most likely the most important structure to control, as you can easily relocate your forces lightning fast. Each star system might or might not have a Nexus Helios connection, and must be found before you can use it.