Problem in reading the "Blue Book"

Hi everyone, i start reading the “Blue Book”, in chapter2, i can’t understand


During the previous stage of the OpenGL pipeline, fragments are converted to pixels in the frame buffer. The frame buffer is actually organized into a set of logical buffers—the color, depth, stencil, and accumulation buffers. The color buffer itself consists of a front left, front right, back left, back right, and some number of auxiliary buffers. You can issue commands to control these buffers, and you can directly read or copy pixels from them. (Note that the particular OpenGL context you’re using may not provide all of these buffers.)
What is the Color Buffer? Why say it contains other buffer, like front right,front left…? Thanks for help in advance.

Normally you use doublebuffering, this means that you have two buffers, one “front” buffer which is the one that is displayed in the window, and the “back” buffer is the buffer you render to, this avoids flickering. “left” and “right” buffers are used when you do stereo rendering, then you need to different buffers, one for left eye and one for right.

But we only render to one buffer at the time so for example the depth buffer only needs to exist for the buffer we are rendering to.