CMYK is the colour model that printers use to create the detailed colour of everything you print. It is also called four-colour process. CMYK stands for cyan, magenta, yellow, and black. Yes, K stands for black. This is because some may think that B stands for blue, so to avoid that, K is used.
Something to think about when printing is that your computer screen uses the RGB colour model to display colours, so what you see on a screen may not be what you see on the printed page. So if you plan to print a lot of something, test it by printing one copy first!
For more on colour, click here!