Humidity is the amount of moisture in the air. There are different ways to express humidity. I will go over the two most common ways which are the relative humidity and the dewpoint.
The relative humidity is the amount of moisture in the air as a percentage of the most moisture that could be in the air at a certain temperature. If the air has half the amount of moisture it could have then the relative humidity is 50%. When it is raining or snowing and the maximum amount of moisture has evaporated into the air then the relative humidity will be at 100%. The relative humidity tends to be highest in the morning since cooler air can not evaporate as much moisture as warmer air. In weather forecasting, the humidity aloft is often more important than the humidity at the surface. When the relative humidity is 100% through a deep layer of the atmosphere and air is rising then precipitation will occur. A relative humidity above 80% will feel humid especially in mild or warm air. A relative humidity below 30% will feel dry.
Another popular expression of humidity is the dewpoint. This is the temperature that the relative humidity will be 100% when air is cooled. When air cools the relative humidity will increase. It will continue to increase as the air cools until it reaching the saturation point. Morning dew is common in humid places in the morning. This occurs from air cooling to the dewpoint and further cooling results in condensation. The temperature will always be greater than or equal to the dewpoint. When the temperature is equal to the dewpoint then the relative humidity is 100%. The dewpoint is a good indicator for noting how dry or muggy it feels outside. Dewpoints of 60 F or higher will feel muggy. Dewpoints less than 40 F along with the temperature greater than 70 F will feel dry. Dewpoint is a popular measure in weather forecasting since it is used to be able to tell how much moisture a storm could have. Warm and high dewpoint air will produce storms when the atmosphere is unstable and rising air occurs.