I'm not sure what is confusing, it's a pretty straightforward paragraph. If you have a mixture of 2 compounds, the vapor phase will have a higher mole fraction of the more volatile compound.
The second part states that if you add a liquid that increases the vapor pressure of the liquid, that is the one that will have a higher concentration in the vapor phase.
Can you clarify what exactly about the paragraph is confusing, as it seems like a pretty simple paragraph...