In Star Trek Generations, the extradimensional energy ribbon is called the Nexus. It is a vast, mysterious phenomenon that travels through space and periodically sweeps across star systems, carrying ships and people into an otherworldly realm.
The film opens in 2293 with the maiden voyage of the USS Enterprise-B. The ship encounters the ribbon while rescuing El-Aurian refugees aboard the Lakul and another vessel. Although the Enterprise-B escapes, James T. Kirk is apparently killed when the ribbon damages the ship. He is actually drawn into the Nexus, where time and ordinary physical rules do not work normally.
Decades later, Dr. Tolian Soran becomes obsessed with returning to the Nexus. He destroys stars with trilithium weapons so their altered gravity will change the ribbon's course and bring it to Veridian III. His plan threatens millions of people, forcing Jean-Luc Picard to intervene.
A common mix-up is treating the Nexus as only the visible ribbon. In the film's terminology, the ribbon is the gateway or transport phenomenon, while the Nexus itself is the paradise-like extradimensional realm reached by entering it. Picard and Kirk ultimately use that realm to travel back and confront Soran.