The curved lines represent the degree of saturation of the air, or relative humidity, with the upper left representing saturation.
As an example of how this is read, if we have a dry bulb of 90°F and a wet bulb of 85°F, our relative humidity is about 80% and the absolute humidity is a bit above 0.024 pounds of water per pound of dry air. If you follow that "humidity ratio" line to the left, you'll also see that the saturation temperature, or dew point, is at a dry bulb temperature of about 82°F, meaning that if all else in constant, you will experience condensation if the temperature falls below that.