CMYK stands for Cyan, Magenta, Yellow, and Key (Black). It is used when projects need to be printed physically and not viewed on a screen. A printer uses these 4 basic tones to create other colors. CMYK colors are known as subtractive colors. As more and more colors are mixed, the resulting tone gets darker ultimately leading to black.
The CMYK color system is a popular choice for packaging as it consumes less ink overall. Very often, your packaging suppliers will require you to send your designs in a packaging dieline format. It is a good practice to start your designs in the CMYK color mode instead of RGB. Changing the format midway may result in the colors displaying differently since CMYK tones are much lighter than RGB tones.