RGB mode refers to the additive colors Red, Green and Blue. It is optimized for display on computer monitors and peripherals, most notably scanning devices. If red, green and blue light are combined, you will get white light. This is the principal behind the monitor your computer uses.
CMYK mode refers to the subtractive colors Cyan, Magenta, Yellow and Black. If we print cyan, magenta and yellow on white paper, they absorb the light shining onto the page. Since our eyes received no reflected light from the paper, we perceive black. The printing world operates in subtractive color, or CMYK mode.
in practice, printing subtractive inks may contain impurities that prevent them from absorbing light perfectly. They do a pretty good job with light colors, but when we add them all together, they produce a murky brown rather than black. In order to get decent dark colors, black ink is added in increasing proportions, as the color gets darker and darker. This is what the "K" stands for in CMYK. "K" is used to indicate black instead of "B" to avoid possible confusion over Blue ink.