The maillard zone is the stage at which browning and color change beginnings to occur in the beans. During this stage, sugars and amino acids begin to break down, which essentially also develops the acidity in your cup. Ideally, this phase isn’t rushed and is typically done with time in an effort to truly maximize flavor development from the chemical reactions that are happening.
Ideally, the roasting curve that is developed should take into consideration the timing of the maillard phase. By the end of the maillard phase, the browning will have finished (moving from its green color to a yellow color and then to the brown color) and the first crack will occur. This is a sign of development and an indicator that the bean has begun opening up and becoming less dense.