Humidity is moisture in the air. When air feels damp, the humidity is high. When air feels dry, the humidity is low. The amount of water the air can hold is determined by the temperature.
Warm air can hold much more moisture than cold air.
When air can hold more moisture at a given temperature it is said to be saturated. Relative humidity is the percentage of saturation. A psychrometer is one tool for measuring relative humidity. The bulb, or rounded end of one thermometer is covered with a wet cloth. After the thermometers are fanned the wet bulb temperature will usually be lower that the dry bulb.