Just like any other thermometer, a stove thermometer also measures the temperature. But the design of a stove thermometer is optimised for using near fireplaces. Depending on the type of thermometer, the exact working of it varies. But basically, you put it near the stove with its probes facing towards the fire. After a while, the dial on the thermometer will give you the temperature of the fireplace. Usually, you have to put it right in front of the fireplace just outside the screen cover. Although the reading on such a thermometer will never be precise, it is enough to give you an idea about how hot the fireplace is.