High humidity is generally regarded as the dew point temperature being at or only a few degrees below the measured air temperature. Dew point is the temperature at which water vapor condenses on surfaces, such as when morning dew condenses on grass.
Humidity measures the amount of water in the air; specifically the amount compared to the maximum amount. The higher the air temperature, the more water can be contained in the air. There are two ways in which the relative humidity can increase with all other variables remaining equal. First, the temperature can drop while the water content remains the same. The second way humidity can increase is if water is absorbed into the air while the temperate remains the same.
High humidity can cause discomfort for humans; it prevents efficient evaporation of sweat, which can cause issues with keeping cool. For plants, high humidity can interfere with transpiration, evaporation from leaves and stems, and cause overheating similar to what humans experience. High humidity can also foster diseases in plants, particularly fungi and molds that thrive in high-humidity environments.
Low humidity can also cause issues; humans may not recognize that they are sweating and become dehydrated. Some harvested fruits require high humidity to prevent shriveling.