Answer:
A dyson sphere is a hypothetical structure, it is thought of as a way in which an advanced civilization could harness the energy of a star.
Detecting dyson spheres can be a way to find signs of life in space, since they are a sign of a civilization capable of contructing such mega structure.
The problem with detecting them is that dyson spheres surround the star, making it less visible for telescopes on Earth.
But the spheres would radiate infrared radiation, so they could be detected by infrared detectors.