Relative Humidity. Relative to What??

Relative Humidity (RH) is the amount of water in the air relative to the maximum amount of water the air can hold at that temperature.
If the air is 70 degrees and has a 60% RH, then the air is 60% full of water compared to the maximum amount of water 70 degree air can hold. The magic trick that keeps things interesting is that when you cool air, it shrinks and can't hold as much water. So if we cool the air without taking any water out of it, then the RH goes up, even though we have not added any water.

PICTURE THIS:
You're holding a plastic water bottle that is 60% full. When you squeeze the bottle and make it smaller, the water level goes up. The opposite is true when we heat the air without adding water. The RH goes down. For every degree we cool the air, we raise the RH by 2.2% provided we don't add or take any water out of it.