"The curve" below maps quality over time that can be applied to several concepts. For example ../../grinding and ../../sourcing-beans.

        |              
        |-             
        | -            
        | -            
        |  -           
        |  -           
        |   -          
QUALITY |   -          
        |    -      |
        |     -     |
        |      -    V 
        |       --     
        |         --------------------________
        |_____________________________________
                          TIME

The general idea is that most coffee consumed by the average drinker is consumed well "after the curve" (denoted by the arrow).

After this point, the volatile flavors of the roast have been lost and the grind has become stale from overextraction.

After the curve, the decreases in quality do not change as dramatically. Because this is the nature of coffee most people are used to, it's easy to do most things right only to be off in the critical timing of preparation before the curve.