Dewpoint and humidity can both be used to describe the comfort level during summer's heat. There is a difference between them. Humidity is a calculated value and varies significantly throughout the day as the air temperature changes. Dewpoint is a temperature measurement. It is the temperature at which air must be cooled for saturation to occur.
A dew point is a better indicator of the actual water vapor content in the air. The more water vapor the higher the dewpoint temperature. The dewpoint and air temperatures are closest his computes into a high relative humidity. The bigger the difference the lower the humidity and more comfortable we feel.
Why does water vapor makes us feel hot and uncomfortable? The answer to that has to do with how the body cools itself. The human body cools itself through evaporation of perspiration. When the temperatures and dewpoints are high, the air is nearly saturated and the perspiration cannot evaporate quickly. This means less cooling for the body so we feel warmer and in some cases very uncomfortable.