A hygrometer measures the relative humidity of the air around it. Humidor hygrometers are either analog (a dial and needle moved by a material that reacts to moisture) or digital (an electronic sensor with a numeric display, often with temperature too). Neither is reliable until it has been checked, so calibrate a new one with a salt test or a calibration kit and check it again from time to time. Place it near the cigars rather than right beside the humidifier.