It is a checkbox on the graphic options tab - where Ambient Occlusion and Motion Blur are. It only effects visual if you are also running "High" overall Graphic settings. (Medium or Low already have it turned off it would seem) It makes very bright light sources less 'glowy' and removes the halo around them. Performance wise - no idea, probably a GPU based effect ?
It makes them much easier to use on Esamir, though obviously you still have white infantry on white snow. It also helps with the massive glow that centralized hud has as well.