Explain how air becomes saturated?

Air can mix with other gasses forming a solution just like water forms a solution with salt or sugar. With a liquid solution there is a limit to how much the water can absorb, and the maximum amount it can absorb depends on the temperature of the water. Solutions in air work the same way. As water evaporates, the water molecules can form a solution and be dissolved in air.

The maximum amount of water molecules dissolved in air depends on temperature (and air pressure). When the maximum amount is reached, the air becomes saturated with water and a change in temperature or pressure or more water entering the air can cause the water to leave the solution. It can result in fog or clouds forming which can result in rain, snow, etc.