At a certain temperature and pressure, a blob of gas can only hold a certain
amount of water vapor inside. If the temperature drops or the pressure
increases, the maximum water vapor that this blob of gas can hold also drops.
The water vapor starts to condensate on some cold surface as dewdrops. The temperature, at which the water vapor starts to condensate, is called the dew point. It is an absolute measure of the humidity of the gas at certain pressure. The higher the humidity, the easier for the water vapor to condensate (One does not need to lower the temperature a lot to see some dewdrops), and the higher the dew point. The dew point is a function of pressure.
The relative humidity is the ratio of actual amount of water vapor to the maximal amount of water vapor that a blob of gas can hold at a certain temperature and pressure. It is usually expressed in percentage. It is a function of both temperature and pressure. This is because when the temperature changes, the maximal amount of water vapor that the blob of gas can hold also changes, i.e., the reference changes, this causes the change of the ratio, which is the relative humidity.
At the atmospheric pressure, the dew point can be calculated from the relative humidity and temperature: