In printing terminology, CMYK stands for Cyan, Magenta, Yellow, and Key (Black). These letters stand for the four colors that are used in the printing process.
CMYK is one of the two commonly used color modes in your design editing software like Photoshop. The other one being RGB, which stands for Red, Green, and Blue.
When submitting artwork to be printed, we highly recommend that you send your design already in CMYK mode. Since computer monitors show colors that are based in the RGB color mode. When printed, your artwork will be converted to CMYK mode, so the colors of the final printout may not match your original. To avoid any problems with this, set your color mode to CMYK before submitting your file for printing. This will ensure the colors you intend come through when your design prints.