It's our tendency to buy bigger, better, and nicer things as our income rises. It describes a very reliable way that human behavior changes over the long-term. It happens slowly and can be applied to other areas of life when deciding to make a change.
Choices can be well reasoned or poorly reasoned. Their results can be surprisingly beneficial or surprisingly damaging, but there's no right choice, just an array of possible ones—and for each, an endless web of consequences.