If you’ve ever printed anything with a commercial print service, you’ve likely heard the term CMYK. The acronym stands for Cyan (blue-green), Magenta (deep purplish red), Yellow, and Key: the colours used in printing presses to make up images from four specific colour plates of ink dots. It's called 'Key' because black is considered one of the primary colours; we call them all by their acronyms so that they're easier to remember when designing prints or laying out files properly before sending off projects!
Most printers use CMYK because it's easier to standardise than RGB. Not only is the colour spectrum for this type of printer more uniform, but there are so many minute variations in colours that they can't afford to be consistent with every print run and between different runs.
Most commercial printers will print in CMYK instead of both types (RGB) because printing in this format makes consistency much simpler throughout a whole production process or even multiple productions, which most companies cannot risk when dealing with small products like prints details matter greatly.