Page "Video codec" Paragraph 5

Video codecs seek to represent a fundamentally analog data set in a digital format.

Because of the design of analog video signals, which represent luma and color information separately, a common first step in image compression in codec design is to represent and store the image in a YCbCr color space.

The conversion to YCbCr provides two benefits: first, it improves compressibility by providing decorrelation of the color signals ; and second, it separates the luma signal, which is perceptually much more important, from the chroma signal, which is less perceptually important and which can be represented at lower resolution to achieve more efficient data compression.

It is common to represent the ratios of information stored in these different channels in the following way Y: Cb: Cr.

Refer to the following article for more information about Chroma subsampling.