RGB refers to the primary colours of light—Red, Green, and Blue—that are used in monitors, TVs, and computer screens; CMYK refers to the primary colours of ink pigment—Cyan, Magenta, Yellow, and Black—that are used in four-colour process printing.
RGB colour combinations cannot be exactly reproduced if your file is converted to CMYK via automated preflighting. As a result, if your artwork was created in RGB, we strongly recommend that you convert your file to CMYK before uploading. After you convert your files to CMYK you may need to make colour corrections to the file to match your colours to the original colours you desired.