Both are color models. They are the two models you mostly use when you're doing any design work.
CMYK (aka Cyan, Magenta, Yellow, Black) is a subtractive color model (reflected light color model). As we add ink to a white page it gets darker. The more ink you add, the page gets darker and turns black. It is used for printed projects.
Whereas RGB (a.ka. Red, Green, Blue) is an additive color model. Unlike CMYK, When you add the RGB colors and overlay those colors you'll get white. For digital designs, you can use RGB colors. It is solely for on-screen use. Therefore, RGB colors are brighter than CMYK colors.