The Maillard reaction is what happens when heat forces amino acids and sugars on a food's surface to react with each other, building hundreds of new molecules that taste and smell like nothing either ingredient started with. It's why a seared steak's crust tastes nothing like the gray meat underneath, why toast smells like toast instead of bread, and why a pale, underbaked cookie never develops the flavor a browned one does. Named for the French chemist who described it in 1912, it's the reason almost everything people call "good browning" happens at all.
What's actually happening on the surface
Amino acids — the fragments proteins break down into — sit right on the surface of raw meat, bread dough, and coffee beans, next to whatever sugars are also present. Left alone at room temperature, they mostly ignore each other. Heat changes that. Above roughly 280°F (140°C), amino acids and sugars start bonding directly, kicking off a chain of further reactions that produces browned pigments called melanoidins along with hundreds of small, volatile compounds — some nutty, some meaty, some faintly burnt.
No single compound accounts for "Maillard flavor," which is part of why it's hard to fake. A seared steak generates different volatile compounds than roasted coffee or baked bread, because each starts with a different mix of amino acids and sugars on its surface. The reaction is the same chemistry every time. What it produces depends entirely on what was sitting there to react.
Why it needs a dry, hot surface
Water caps the temperature a wet surface can reach at 212°F (100°C), its boiling point, and it holds it there until every last drop has evaporated. That's well short of the 280°F the Maillard reaction needs to get started, which is the entire reason boiled, steamed, and simmered food never browns no matter how long it stays on the heat. A stew can simmer for six hours and the meat in it still won't develop a crust — it's never once been hot enough on the surface.
This is also why patting meat dry before it goes in a pan matters more than people assume, and why crowding a pan full of vegetables leaves them gray and steamed instead of browned. A wet surface, or a pan releasing enough steam that it can't escape fast enough, keeps pulling the surface temperature back down toward 212°F every time it tries to climb past it.
Maillard reaction or caramelization
They get lumped together because both brown food and both taste good, but they're separate reactions with a different ingredient list. Caramelization is sugar breaking down on its own, no protein required — it's why a pan of nothing but sugar and water turns amber and toasty by itself. The Maillard reaction needs an amino acid paired with a sugar; a food with almost no protein, like plain white sugar, can't undergo it no matter how hot the pan gets.
Onions are the case that trips people up, because "caramelized onions" is the name everyone uses for a dish that isn't purely caramelization at all. Onions carry a meaningful amount of protein alongside their sugar, so a slow-cooked pan of onions is running both reactions side by side — which is a real part of why they taste deeper and more savory than sugar syrup ever will, and not just sweeter.
Why alkalinity speeds it up
Raising a food's surface pH accelerates the Maillard reaction, which is the working principle behind a few kitchen tricks that otherwise look unrelated. A pinch of baking soda stirred into sliced onions gets them a deep amber color in a fraction of the usual time, because the alkaline surface reacts faster at the same temperature. Pretzels get their glossy mahogany crust from a quick dip in a lye or baking-soda bath before baking — the surface is alkaline before it ever sees the oven's heat, so the crust browns hard while the interior stays pale dough.
The same lever runs the other way, too. An acidic marinade lowers surface pH and can measurably slow browning, which is one reason a heavily acid-marinated piece of meat sometimes takes longer to get real color in the pan than an unmarinated one does, even at the same heat.