A relative humidity of 50% means that twice as much air is evaporating at any given time, than going from air vapor to liquid water. As the humidity drops, the evaporating increases, and vice versa. If the humidity is 100%, you have saturated air -- equal evaporating and condensation, at which point a body of water won't evaporate. Thus, low humidities signal dry air, and vice versa.
If you have more condensation than evaporation, you have a humidity of higher than 100% (called super-saturation), which is how clouds form.