Evaporation is the process of water changing from the liquid to vapor state. If the air is already saturated with water (extremely moist air), when more water evaporates some of the water that is already in the air condenses (goes from vapor to liquid). So there is no net evaporation.
But, if the air is far from being saturated, it can hold a lot of water. The body of water just keeps evaporating. The drier the air, the more water can evaporate. (Until the air is saturated.)