What we call humidity, also called relative humidity, is a measure of how much water vapor the air can hold. The air we breathe has water in it in the form of water vapor. The higher the temperature the more waver vapor the air can hold, the lower the temp the less water vapor it can hold. When you have 100% humidity it means the air has all the water vapor it can hold at the current temp. Say the temp goes down with all the other variables remaining the same, the air needs to "shed" water (in the form of dew, mist, fog, or even rain) so that it can reach back to its point of equilibrium at 100%. If the temp goes up with all the other variables remaining the same, then the air now can hold more vapor than it currently holds, and humidity is now less than 100%, say 80%. In other words the air is currently holding 80% of the vapor it could otherwise hold at the current temp.
Dew point: if in the current conditions you bring down the temperature with all the other variables remaining the same, dew point is the temperature at which you would reach 100% humidity. In your case, say you are at 45% humidity at 30 F, a dew point of 4 means if you were to magically bring down the temp to 4F without changing any other variables (wind, etc) you would then be at 100% humidity.
One interesting thing in relative humidity vs. temperature is that it varies during the day depending on temp. At night it may be close to 90% and during the day when the temp goes up and not much other weather events occur, the humidity goes down mainly due to temp, although other events like wind, etc can also affect it.
Here's a humidity forecast plot from national weather service for Boston (green line on third plot):
Hourly Weather Forecast for 42.36N 71.07W (Elev. 13 ft)
Hope it makes sense.
