Tone reproduction theory. In broad terms, how the luminances and luminance relationships of the original scene are ultimately rendered in print values. The quadrant diagram Stephen refers to is a graphical way of following the original values in the scene through the various steps to the final print. There are variables, controls and limitations along the way that change these relationships. The general idea is if you really want to "predict" how the scene will look in the print (Zone System, for example), you need to look at the entire system from subject to print. In other words, it is not enough to know your film curve. There is also the paper curve, flare etc.
This particular thread goes a step further than the objective transition from subject to film to print, and adds subjective aspects like the conditions under which the print is viewed, the brightness of the background, the way our eyes react to value relationships in the print.