What makes this all tick is that warm air can hold more water than cold air. When it is hot our body cools itself off by sweating. The THI is a formulaic way of describing this phenomena.
Humidity is a measure of moisture content of air.
The dew point is a measure of absolute humidity. There are several measures of water vapor. This puts a large strain on the body. Argonne National Laboratory, Division of Educational Programs, Harold Myron, Ph.D., Division Director.