A power node was an energy-production device.
In 2375, the crew of the Federation starship Voyager managed to appropriate two power nodes and a dozen plasma conduits from a Borg probe they had destroyed. Later, while trapped in the Borg Queen's chamber aboard the Unicomplex, Captain Janeway destroyed the Queen's power node with a phaser rifle, disrupting the Queen's command interface, preventing her from adapting to Seven of Nine's attempt to shut down a dispersal field the Queen was using to prevent the Voyager crew from obtaining a transporter lock on Janeway and Seven. (VOY: "Dark Frontier")
The next year, Neelix appropriated a power node with transkinetic energy regulators and bioneural control circuitry from a merchant named Onquanii, who had himself acquired the node from a species called the Druoda. Later, when Voyager fell under the control of a series 5 long-range tactical armor unit, also constructed by the Druoda, Neelix noticed the similarities between the two technologies, allowing the crew to approach Onquannii for help. (VOY: "Warhead")