Oven Cord Meat Thermometers are three piece thermometers. A probe that goes into the joint of meat that you are roasting in the oven, a “reader module” that stays outside the oven and reports the temperature that the probe is detecting, and a cable that connects the two which conveys the information from the probe to the reader.
The advantage is that you don’t have to open the oven to check the temperature, which lets the heat out, or have to go to the hassle of heaving the turkey out in a crowded kitchen.
With most of these models, you tell them what temperature you want to reach and then they will beep when they’re there.
See main entry for Meat Thermometers.