This short section will try to answer the following questions:
Why it is possible to saturate air with water vapor?
Why is there an upper limit to the amount of water vapor that air can contain?
Why does this upper limit depend on air temperature?

First we need to understand that the rate at which water evaporates depends on the water's temperature.
You can test your intuition.  What would evaporate more quickly, a glass of iced tea or a glass of hot tea?

Hot water evaporates more readily than cold water.   In the figure above (top of p. 84 in the photocopied Classnotes) the hot water is evaporating three times more rapidly than the cold water.

To be able to evaporate, a water molecule in a glass must make its way up to the surface of the water and the water molecule must then have sufficient kinetic energy (to overcome any attractive forces trying to keep it in a liquid state).

The distributions of the kinetic energies of the water molecules in the glasses of cold and hot water (remember temperature is a measure of the average kinetic energy) are shown in the two graphs above.  In cold water only a limited number of the water molecules (those to the right of the highlighted line) have the necessary energy - cold water has a low rate of evaporation.  In hot water, the whole distribution has moved to the right, the threshold energy needed to evaporate has remained the same, but a larger fracton of the water molecules have moved to the right of that threshold.  Hot water evaporates more rapidly.

Now we will look at the top part of p. 85 in the photocopied notes.  We have put a cover on the glass of room temperature water.

In #1 we see that the water is evaporating (2 blue arrows worth of evaporation).  Water vapor will begin to build up in the air in the glass.  This is shown in #2.  Some of the water vapor molecules will condense (molecules that find themselves with lower than average kinetic energy).  In Fig. #3 the amount of water vapor has built up to a point where the amount of condensation is becoming significant and one orange arrow worth of condensation has been added to the picture.  In #3 there is still more evaporation than condensation so the water vapor concentration will increase a little bit more.  Eventually in #4 the water vapor concentration has increased to a point that there are two arrows of condensation.  This balances the 2 arrows of evaporation.  The air is saturated, the air is filled to capacity.  With equal rates of evaporation and condensation, the amount of water vapor in the air will now remain constant. Note that the relative humidity is 100% at this point.

What would happen if we took off the cover and added some more water vapor to the glass in Fig. #4?

The air in Fig. #5 shows what would happen.  The air would be supersaturated with water vapor and the RH would be greater than 100%.  This is possible but it is not an equilibrium situation and wouldn't remain this way.  The increased amount of water vapor would increase the rate of condensation.  There would be more condensation than evaporation (3 orange arrows of condensation and 2 blue arrows of evaporation in the figure above).  The water vapor concentration would begin to decrease.  Eventually the glass would return to the equilibrium situation in Fig. #4.

We bring back the glasses of cold and hot warm at the bottom of p. 85 in the photocopied Class Notes.

The relative humidity is 100%, and the air is saturated in both cases.  Not much water vapor is needed to provide 1 arrow of condensation needed to balance the 1 arrow of evaporation in the cold glass.  There are 3 arrows of evaporation in the glass of warm water.  There must be a lot more water vapor in the air to provide 3 arrows of condensation.