A thermocouple is used to measure temperature. It works by generating a small electrical voltage (known as the Seebeck effect) when two different metals or alloys are joined together at one end and heated. The voltage produced is related to the temperature difference between the hot junction (the part that's exposed to heat) and the cold junction (the reference point, usually kept at a known temperature).
Thermocouples are widely used in various industries to measure temperatures, especially in harsh environments, because they are durable, inexpensive, and can measure a wide range of temperatures. They are commonly used in things like furnaces, engines, ovens, and industrial processes.