What Do These Letters Refer To?
CMYK and RGB are color modes and represent the colors used to produce an image. CMYK refers to Cyan-Magento-Yellow-Black, and RGB refers to Red-Green-Blue


RGB Color Mode
RGB is an additive color mode associated with electronic displays such as computer moniters, TV screens, tablet and phone screens, etc. RGB combines the primary colors of red, green, and blue to create an array of colors. RGB offers a wider range of colors that are brighter, more vivid
and more vibrant than CMYK can create. Your TV, computer and phone screens use this mode
because of this.

CMYK Color Mode
CMYK is a subtractive color mode associated with printing. CMYK combines the colors of cyan,
magenta, yellow, and black in various amounts and mixtures to create a variety of colors to
produce and print an image to paper.

Converting RGB Colors to CMYK for Printing
As mentioned before RGB color mode has a wider range of colors that are brighter and more vivid than CMYK. CMYK cannot produce these vibrant colors, and so if you try to print an image that uses the RGB color mode using colors outside the range of CMYK, the colors printed on the paper will appear more dull than what you view on screen. It's important to convert an RGB image to CMYK before printing. This will give you a more accurate representation of how the image will print on paper. See the below examples. Any photo editing or graphic design program should have the ability to convert RGB images to CMYK.

Example 1 

Example 2
RGB vs CMYK Image