Power nodes are a new feature in Natural Selection 2. They control the lighting for rooms and power for marine structures. Their use is a prime example of the Spark engine's dynamic lighting capabilities.
When a power node is damaged, the lights in a room are dimmed. Also, marine structures cannot receive the crucial energy they need to power their many structures. This means infantry portals cannot spawn reinforcements, armories cannot dispense ammo, etc. Marines end up literally fighting for their lives.
In order to repower their structures, the marines must weld any damaged power nodes. This can be done by commander-controlled builder bots called MAC's. Once a power node is repaired, lights in the room turn back on and structures come back to life.
This new concept increases strategy and forces both teams to fight constantly over territory, since controlling power nodes effectively means control of the room. To demonstrate this concept more clearly, NS2HD has posted a very nice video.