Sorry, the HVAC engineer in me spilled out for a moment. A psychrometric chart is a chart that tracks the moisture capacity of air at different temperatures. It's rather odd looking (think of a square but with the left corner cut off in a curve) - lines rising straight from the bottom are dry bulb temperature (the temp you read with a thermometer), lines running diagonally from top left to bottom right are wet bulb temperature, curves running from bottom left to top right are relative humidity curves. The point where wet bulb and dry bulb intercept is the dew point. Toss in a few more lines, and you get a really confusing mess. On the bright side, it is possible to tell a lot about the temperature, moisture content, and changes that take place to both if you are familiar with it.

The reason that it all works is that a pound of air can hold a specific amount of water, but that specific maximum changes a lot as you change the temperature of that air. If you dump a spoonful of water into a pound of 70F air, it should be able to hold all of it and have room for more, but if you cool that air to 50F the maximum moisture content changes and the spoonful no longer fits. When that happens, some of the water falls out - that's condensation. Dew point is the point at which the air cools to the point where it is completely full of water, such that cooling it any more will cause water to start falling out. Relative humidity is a measure of how full the air is - 40% RH means the air is holding 40% of its maximum capacity.