Stands for Cyan, Magenta, Yellow and BlacK. The 4 printing colors. Black is indicated by “K” instead of “B”, which might seem a bit odd.
Often this is explained by the story that the “K” is chosen to prevent mix up with Blue. This is not correct however.
This usage of the “K” instead of the “B” for Black comes from traditional printing where the black color printing plate was referred to as the “Key plate” or “Key color”. Hence the “K”.