Dew point is the temperature at which the moisture in a gas will condense. More specifically, it is the temperature at which the va-por pressure of a gas equals the saturated vapor pressure. As it relates to drying plastic pellets, the gas in question is air. The more parts per million of water particles in the air, the higher the vapor pressure, and consequently, the higher the dew point. As the dew point of air is reduced, its ability to take in moisture is increased.
The difference between dew point and relative humidity as a measurement of moisture in air is that relative humidity changes as the air temperature changes, but the dew point stays constant with temperature changes. Dew point is a measurement of actual water content. Relative humidity changes with temperature, even with the same amount of moisture in the air, because the density of air changes with temperature.