What does the weather forecast mean by air humidity? Why is there no air humidity of 100%?

The air humidity in the weather forecast refers to relative humidity, not the ratio of water to air! Is the amount of water contained in the air, accounting for the percentage of the air's ability to dissolve water.

When the humidity reaches 100%, it is saturated, which is difficult to reach at low altitude.

However, if the low-altitude gas rises to the high altitude for some reason, the temperature drops, and the ability of air to dissolve water decreases, it will reach saturation (reaching 100% humidity at a lower temperature), and clouds will cause rainfall.