Definition: the browning reaction between amino acids and sugars under heat that creates deep, toasty colour and flavour.
The Maillard reaction (named after the French chemist Louis-Camille Maillard —my-yar) is why a golden cookie edge tastes so much better than a pale one. It’s not the same as caramelisation, which is sugar alone; Maillard needs both protein and sugar, which is why an egg wash browns, and why a splash of milk or a raising agent that lifts the pH speeds it along. It’s the difference between “cooked” and “delicious.”
In practice: bake to a proper golden edge, not a nervous blond — that colour is flavour.
Related:bicarbonate of soda,crumb.