A thermocouple is a temperature measuring device that is exposed to a process in order to determine its temperature. A thermowell is a structure that surrounds the thermocouple (or RTD) probe and protects it from aspects of the process such as fluid flow rates or caustic or degrading materials.
The standard length from the sealing face of the cap and the face of the temperature sensor end of the well is 1-3/4”. "U" is the insertion depth of the thermowell. This is how much of the thermowell will be inserted into the process, and is measured from the cap face to the tip of the well.