It is a good idea to calibrate your meat thermometer, especially if you haven't used it in awhile, or have dropped it since you last used it. There are two ways to calibrate a meat thermometer: at freezing point and at boiling point. To calibrate it at freezing point, completely fill a glass or pot with ice, and then add cold water. Use crushed ice if there is some available. Let the glass stand for a few minutes (to thoroughly chill the water to 32 degrees), then use your meat thermometer to measure the temperature of the ice water. If it doesn't read 32 degrees, your thermometer needs to be calibrated.
To calibrate at the boiling point, fill a sauce pan with water, then bring it to a boil on your stovetop. Insert the thermometer into the center of the water (in every direction, so it's neither near the surfaces of the pot nor the top of the water). At sea level, water boils at 212°F, and that's what your thermometer should read. If you are above sea level, water boils at a lower temperature. For example, at 8000 feet, water boils closer to 196°F. If you are calibrating your thermometer using the boiling point method at a high altitude, use a reliable source to calculate the boiling point of water in your locale before performing the test.
If your calibration test reveals your thermometer to be out of alignment, adjust the nut on the backside of the thermometer with a small screwdriver, then test it again. For an electronic thermometer, see the manufacturer's instructions on resetting the thermometer. Usually there will be a button you can push while it is at freezing or boiling temperature to perform the calibration.