Very simple. There are based on the Seebeck effect where any two dissimilar metal wires welded at one end will generate a millivolt DC voltage proportional to the heat. K type uses metal wires of Iron and Constantan.
Advantages: Low cost, rugged, very high temp range.
If you don't require the very high temp range of a TC there are more modern and more accurate temp sensors, RTD, solid-state, etc that don't require as much support circuitry.