Why does toast turn brown?

Toast turns brown because heat makes sugars and proteins in bread react with each other. That reaction creates new brown bits, smells, and flavors on the surface. It is called the Maillard reaction, and it happens before the bread burns.

The same browning shows up on baked crusts, roasted potatoes, and many foods cooked with dry heat. Leave the bread too long, though, and the tasty browning changes into black char, which is why burned toast tastes sharp and bitter.