The Maillard reaction is technically “nonenzymatic browning,” which basically includes any kind of browning that happens when you’re cooking, but excludes what happens to cut apples you leave out on the counter. At the chemical level, it’s a reaction between an amino acid and a sugar in the presence of heat. A huge number of chemicals are formed in these processes, so you can’t really pin down the Maillard reaction to a single set of chemical steps. These processes are responsible for the browning of meat, the malting of barley for beer, the roasting of coffee, and the browning of the crust of bread.