Disposable meat thermometers (aka single-use thermometers) don’t give you exact cooking temperatures but tell you what range you are in.
You take your meat out of the oven or off the heat, poke a sensor in and get your reading. You insert the sensor and wait anywhere from 5 to 10 seconds. The tip changes colour to tell you rare, medium or well-done. If meat needs more cooking, you remove the sensor, cook the meat more, then poke the same sensor back in. They are generally reliable for two or three uses in cooking the same piece of meat. However, with some makes, once the colour changes that’s it: they can’t be used any further. They are definitely meant to be discarded after one cooking session. Don’t leave them in food while it is cooking.
They are made from materials that the American Food and Drug Administration (FDA) has approved as being safe. Some are cardboard and require you to cut a slit into the meat for them to be inserted into. The plastic ones are like long plastic toothpicks or skewers. Most kinds come in a small plastic jar.
There are different versions for different meats: e.g. one for poultry, and one for all meat including poultry. The poultry ones read partially-done and done.