YCbCr

Luma (brightness-like) separated from two chroma (color-difference) channels, the basis of video and JPEG chroma subsampling.

This course's YCbCr uses the full-range BT.601 form $Y=0.299R+0.587G+0.114B$ , $C_b=128+0.564(B-Y)$ , $C_r=128+0.713(R-Y)$ . Because human vision resolves fine spatial detail in luminance far better than in color, video and JPEG subsample $C_b,C_r$ at lower resolution than $Y$ (e.g. "4:2:0"), cutting data roughly in half with little visible loss -- a saving with no equivalent in plain RGB, which has no channel that concentrates "brightness" the way $Y$ does.