A Dyson sphere is a hypothetical megastructure that encompasses a star and captures a large percentage of its power output. The concept is a thought experiment that attempts to imagine how a spacefaring civilization would meet its energy requirements once those requirements exceed what can be generated from the home planet’s resources alone. Because only a tiny fraction of a star’s energy emissions reaches the surface of any orbiting planet, building structures encircling a star would enable a civilization to harvest far more energy.
Eh, there’s only one in all of Star Trek, and they forgot about it after one episode. Should have a whole series.
There was a star trek novel dealing with it. I read it but don’t remember any details. My favorite along those lines was the ringworld books.