100% Humidity
I was curious, so I went and found this out. It is actually a factor of how much water is in the air, as compared to what the air can hold. The air can hold different amounts of water - the hotter it is, the more water it can hold. So, humidity is directly linked to temperature. The same amount of water in the air at a higher temperature would mean a lower humidity.
It's measured by a special thermometer, which actually has two thermometers in it. It's the difference between the two that counts for the humidity factor.
Regardless, 84% is pretty high, considering 100% means condensation occurs (that's when the air can't hold the water anymore).
Mostly, the scale is an indicator of how much you sweat while walking about, and also how much you will fail to impress people when showing up to meet them sopping wet.