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.
It’s a Stellaris reference. Once you can build Dyson spheres there’s a chance your (also FTL capable) neighbors complain about you blocking their sacred star.
I usually just give them money and they forgive me.