The chemical reaction between amino acids and sugars that browns your bread's crust and creates hundreds of flavor compounds.
The Maillard reaction kicks in above 150C (300F) and is responsible for the complex, nutty, caramelized flavors in a well-baked crust. Long-fermented sourdough has more available sugars (thanks to amylase breaking down starches) and more free amino acids (thanks to protease), which means more intense Maillard reaction and better crust flavor. This is one reason sourdough crust tastes so much better than quick-rise bread crust.
