Humidity is the amount of water vapor present in the air. It is high during the rainy season. The air, however, can hold only a definite amount of water vapor at a given temperature.
Therefore, when the humidity is already high as in the rainy season, the air is either saturated or can hold only a little more amount of water vapor. This slows down the rate of evaporation. This is the reason why we perspire more in hot and humid weather since sweat does not evaporate easily from our clothes and body.