Relative humidity (RH) (expressed as a percent) also measures water vapor, but RELATIVE to the temperature of the air. In other words, it is a measure of the actual amount of water vapor in the air compared to the total amount of vapor that can exist in the air at its current temperature. Warm air can possess more water vapor (moisture) than cold air, so with the same amount of absolute/specific humidity, air will have a HIGHER relative humidity if the air is cooler, and a LOWER relative humidity if the air is warmer.
Not to get too complicated but cold air is denser than warm air and it can hold less water vapor, hence RH goes up if the water vapor is present. Warm air is less dense and can hold more water vapor, hence lower RH because of rapid movement of water molecules and faster evaporation.