When the relative humidity is 100%, the air is saturated with water. This means that the air cannot hold anymore water. When this happens, we will find that our sweat will not evaporate. Human beings sweat and get a cooling sensation when the sweat  evaporates. However, at 100% relative humidity, evaporation of sweat will not happen and we will feel very hot. That is why we like lower relative humidity, when the sweat can evaporate readily and keep us cooler. Another aspect of 100% relative humidity is the potential for precipitation. If the air at 100% relative humidity rises up, its temperature will drop and it will no longer be able to hold as many water vapors. If this happens near a surface, we will see water droplets condensing on the surface. If this happens at higher altitudes, clouds are formed and we get rainfall.

Hope this helps.