How does this relate to agriculture?
On a day with a lot of water vapor in the air (high humidity) the potential for latent heat release makes it more likely for convective showers and thunderstorms to develop. This is part of the reason that thunderstorms can pop up around the Southeast on warm and humid summer days even when there are no fronts in the area. Rain that falls from these storms hits the hot ground and evaporates, which removes a lot of energy from the ground and cools it off. If you are going to apply a herbicide or pesticide which needs to sit on the leaves of your crops for a while to work, then you may have problems on days when the high humidity is more likely to lead to the development of showers. On the other hand, the cooling effect of the rain may feel refreshing on a hot summer afternoon.
Evaporation of sweat in humans and animals also leads to cooling of the skin surface on hot days. Evaporation is more effective when there is some wind to carry the moisture away from the skin area. However, in really hot conditions the body may shut down and stop producing sweat. This is particularly true when you are in areas of bright sunlight, which will increase your temperature due to the sensible heat released by absorbing the solar radiation on your skin. In that case, the body's temperature can rise very quickly and cause hyperthermia or heat stroke, which can lead to death. That is why you need to keep hydrated and take frequent breaks in the shade while working outdoors on the hottest days and seek medical help immediately if you stop sweating in hot conditions.