RGB refers to the primary colors of light, which are Red, Green, and Blue. RGB mode is usually used when images are being viewed on monitors, TVs, and computer screens.
CMYK refers to the primary colors of ink pigment, which are Cyan, Magenta, Yellow, and Black. CMYK mode is what we use to print on our presses.
Please make sure to convert all RGB color images to CMYK to avoid surprises, as there are a few colors that look different in the CMYK color space. for example, Blue looks much richer in RGB color space compared to the CMYK. By default, all pictures taken by digital cameras are in RGB workspace.