The thermowell is a closed end tube that opens to the outside and extends into the center of the fermenter. This lets you insert a thermometer probe from a regular thermometer, or a probe from a temperature controller, so you can pull the temperature from the beer better than affixing something...
I am a new home brewer, and at least every other questions that I search for in google gives me threads from this forum. I figured I should sign up and contribute a bit, since it seems to be an excellent source of information! Looking forward to delving deeper into this hobby!