What is 'CMYK'?
CMYK is an acronym for the colours Cyan, Magenta, Yellow and Key (Key is usually associated with Black). Printing is a complicated process, and this is only a short summary to give you an idea of how CYMK is used in printing.
When you have a project printed via traditional printing, using a printing press, your design is created using the CYMK colours - turning your single image into four separate images. These different images are then printed on top of each other to 'build up' the final image.
For example, the full colour barn image below, printed on a traditional press would have the four different colours printed on top of each other to get the final image: