What is meant by 'combine the luminance and chrominance '
Intensity of color images are defined through color spaces such as RGB, YUV, and HSV. 

RGB is an additive-synthesis primary color decomposition and it's very fundamental. 

Whereas YUV is an example of Luminance (brightness or Black &amp; White) and Chroma (color information saturation and hue) decomposition. Usually chroma consists of two components such as U &amp; V in YUV or H &amp; S as in HSV color spaces.

When luminance and chroma are combined together they create the complete definition, YUV or HSV, of the color information which can be converted into RGB. How they are combined depends on the chosen color space and its conversion into RGB. 

For the case of YUV to RGB conversion, the combination and conversion is as simple as a matrix multiplication.