This 3-in-1 meter is designed to measure the relative humidity (RH) and the soil/plant water potential in one unit. It is also able to measure pH.
It usually has an LCD screen and a touch screen and is a very compact unit that's easy to use, and a very handy tool to have in your garden.
They work on the same principle as an analog thermometer (a little bit more expensive). You insert a probe into the soil, and the meter will indicate the current temperature of the soil, which is the same as the air temperature, minus the water vapor content in the air, plus the humidity in the air. If there is no water in the soil, then the meter will indicate that the soil is at the dew point. If there is water in the soil, then the meter will indicate the temperature of the water.