RGB (AdobeRGB, sRGB) and CMYK are colour spaces that (in simple terms) define what colours are available for you to work with. They’re also referred to as colour profiles.
About CMYK
CMYK has always been associated with print and is based on the process of using Cyan, Magenta, Yellow and Key (Black) to print images and documents.
About RGB
RGB is based on how screens display colours using Red, Green and Blue.