Head-mount temperature transmitter is a device installed directly in the connection head of a temperature sensors, temperature sensor typically will be thermocouple or RTDs, the key feature of temperature transmitter is to convert small and raw signal from the sensor into analog signal like 4-20mA, the transmitter has various options such as isolated type,  Hart version,  Modbus RS-485 compatible version, below is an image on how it looks like and the way it is installed inside the connection head

In many applications, the master device which receive the temperature signal often located at central control room or far away from the filed where the sensor installed, if the extension cable had been used, the signal will be affected by electrial noise and signal will suffer significant attenuation over long cable runs, therefore , by converting the weak signal into 4-20mA or other digital signal will minimizes the negative effects and saves cost too. as the extension cable is much more expensive than the regular cooper wire for 4-20mA,  on the master device such as PLC or digital dispaly, they just to conver the 4-20mA into temperature reading.