When the bread's surface temperature reaches about 310 degrees Farenheit, a chemical change known as the Maillard reaction begins. With more heating, the sugars and underlying grain fibers start turning into carbon. That's burnt toast. As the quote above indicates, the principle of making toast is pretty simple - all you need is radiant heat and bread.
Thermal radiation is electromagnetic waves and includes infrared light.
A hotter object sends more infrared light toward a colder object than vice versa, so the hotter object gives up thermal energy to the colder object. Fire sufficed for ages as the agent for applying radiant heat to bread, but with the coming of the electrical age, things became more complicated - not necessarily for the person toasting, rather for the toasting device itself. In this article, compiled from service and repair manuals from the 1940s and 1950s, the inner workings of a variety of toasters are revealed. If a metal offers high resistance to the flow of electric current, it is not a good conductor. Because of the resistance of the wire, electric current flowing through the wire causes heat.