Fundamentally, an image is a matrix of numerical values. This is why OpenCV 2 manipulates them using the cv::Mat data structure. Each element of the matrix represents one pixel. For a gray-level image (a "black-and-white" image), pixels are unsigned 8-bit values where 0 corresponds to black and corresponds 255 to white. For a color image, three such values per pixel are required to represent the usual three primary color channels {Red, Green, Blue}. A matrix element is therefore made, in this case, of a triplet of values.
Color images are composed of 3-channel pixels. Each of these channels corresponds to the intensity value of one of the three primary colors (red, green, blue). Since each of these values is an 8-bit unsigned char, the total number of colors is 256x256x256, which is more than 16 million colors. Consequently, to reduce the complexity of an analysis, it is sometimes useful to reduce the number of colors in an image. One simple way to achieve this goal is to simply subdivide the RGB space into cubes of equal sizes. For example, if you reduce the number of colors in each dimension by 8, then you would obtain a total of 32x32x32 colors. Each color in the original image is then assigned a new color value in the color-reduced image that corresponds to the value in the center of the cube to which it belongs.