Other comments have said what an SGL is, but don’t properly convey just what one would be capable of. A solar gravitational lens can be used to image relatively near astronomical objects with extraordinary resolution. If we could get a spacecraft to ~500 AU it could be used to observe an exoplanet that is ~100 light years out at a 25 km resolution.

Someone looking at the Earth from 100 LY with that kind of resolution would probably be able to see signs of life.