When our bodies overheat, it begins to sweat to reduce your core body temperature. Once the sweat is on your skin, it should evaporate which in turn will cool your body. However, when there’s more moisture in the air (high dew point) the sweat takes longer to evaporate and the warmer and more uncomfortable you become.
Dew-point is the temperature that air must be cooled to for water vapor to condense (at which the air is 100% moisture). The closer the dew-point is to the actual temperature, the more moisture in the air.
If you place a frozen glass of water on a counter and see moisture develop on the outside of the glass, this indicates dew-point (the temperature that air must be cooled to for water vapor to condense). The cold water will cool the glass to about the same temperature as the water inside. The cold glass will cool the surrounding thin layer of air passing over it. If the temperature of the water/glass is below the dew-point temperature of the air surrounding the glass, the thin surrounding air will condense the moisture it holds onto the glass.
Dew-point temperature cannot be higher than dry-bulb temperature. As dew-point temperature increases closer to dry-bulb temperature, moisture in the air condenses.