This is a test of the camera's noise levels. The higher the signal to noise ratio, the greater the difference in strength between the real image data and random background noise, so the 'cleaner' the image will look. The higher the signal to noise ratio, the better.
Canon EOS-1D X Mk II signal to noise ratio charts
JPEG signal to noise ratio analysis: The 1D X Mk II leads the way for the low to mid sensitivity settings, but the D5 shows its low-light pedigree from around ISO25,600 and pushes ahead, indicating that its images are a little cleaner.
Raw (converted to TIFF) signal to noise ratio analysis: This chart shows that the 1D X Mk II performs well in our lab, beating the Nikon D5 throughout the Canon's sensitivity range. However, real-world shooting shows it's a very close-run race.