This one’s pretty clear – if supply of ‘thing’ is high and demand is low, then the cost of ‘thing’ is low, and vice-versa. What may not be super intuitive is how universally this applies everywhere.
Common sayings – “you only value things when you lose them”, “the grass is greener on the other side” – and so on. The variations are endless. In essence though, they are just applications of how everything is supply vs demand.
The intention of stating the obvious here is to invite active thought in identifying your resources that are high in supply, and to put them to good use, versus waiting for when they may become scarce and we realize their true value.
An obvious example of such a resource, as is a common theme in the rest of the document as well, is time. If you have two hours free on a weekend, put it to good use, even if it’s just going for a walk. The same two hours may become costlier (and hence, more valuable) at a later time.