The Dyson Sphere is a hypothetical megastructure that completely encapsulates a star and captures a large percentage of its power output. This concept is a thought experiment that attempts to explain how a space civilization will meet its energy needs when those needs exceed what can be produced from the resources of the parent planet alone.
Since only a small fraction of the star's radiation reaches the surface of each rotating planet, building structures around the star would allow civilization to receive much more energy.