A fork thermometer is a digital thermometer built into a large fork.
One of the fork tines will have a temperature-sensing device in it. The handle has a scale of bars on it that lights up showing you what range you are in. You usually have to wait up to 10 seconds for the action to happen.
These are designed for people who are grilling (barbequing.)
The downside is that most people who grill would probably say that you shouldn’t be using a fork to turn your meat on a grill anyway: every time you poke it with a fork to turn it, you’re letting more and more juices out. Perhaps it’s better to use another type of thermometer, and tongs, instead.
Do not leave the fork tines in the food while cooking.