Question

What does the CMYK tab let you do

Answer 1

CMYK means cyan, magenta, yellow, and key (black). They are the four color model used in color printing or printing color images. It lets you subtract the colors on the image. CMYK partially or fully masks colors on a lighter view. 

Answer 2

CMYK stands for Cyan, Magenta, Yellow, Key (Black) and is a subtractive shading model. A subtractive color model works precisely inverse from an added substance shading model (RGB,) it works by somewhat or altogether veiling certain hues from white. 
So as opposed to adding light to accomplish a shading, CMYK is utilizing ink to subtract shine from white. Accordingly, Cyan, Magenta, and Yellow joined is black– the nonappearance of brightness. 
CMYK is likewise a device dependent color mode, yet it is most as often as possible used right up 'til the present time as the principle shading mode utilized as a part of printing, regardless of whether it's blurbs, handouts, business cards, books, or magazines.