CMYK means Cyan, Magenta, Yellow en Key.  (Cyan, Magenta, Yellow en Black).

CMYK uses percentages of the colors. This means if you have 100% Cyan and 100% Magenta it would be pure Blue. And if you want a darker Blue you add some Key to it.

CMYK (100,100,0,50) means 100% Cyan, 100% Magenta, 0% Yellow and 50% Black.