what is relative humidity?if the RH is 50%,air contains abouthow much water vapour?

The relative humidity of an air-water mixture is defined as the ratio of the partial pressure of water vapor in the mixture to the saturated vapor pressure of water at a given temperature. Relative humidity is expressed as a percentage   p(H2O) is the partial pressure of water vapor in the mixture; and p * (H2O) is the saturation vapor pressure of water at the temperature of the mixture