These are the analog luminance, Y, and color difference signals (R-Y) and (B-Y) of component video. Y is pure luminance information whilst the two color difference signals together provide the color information. The latter are the difference between a color and luminance:
red minus luminance and blue minus luminance. The signals are derived from the original RGB source, usually a camera.
The Y, (R-Y), (B-Y) signals are fundamental to much of television. For example in ITU-R BT.601 it is these signals that are digitized to make 4:2:2 component digital video, in the PAL and NTSC TV systems they are used to generate the final composite coded signal, and in DTV they are sampled to create the MPEG-2 video bitstream.