I'm not sure how Gimp's color chooser operates, but that means a color that is 50% value and 0% saturation. Or in typical RGB colors: 128,128,128.

Any time all three numbers in RGB are the same, the resulting color will be some level of grey, black (0,0,0) or white (255,255,255). 128 is half of 255, so that's 50% value.