The Kuznets curve is a graphical representation of the idea that as societies advance in technological level from low to high, they become, at first, more unequal. Inequality increases, but after a point, when they reach industrialism and advance to industrial modes of production, the inequality starts to lessen.
The dotted line represents what may happen in the future. A consensus is emerging that our society might be becoming more unequal as it moves towards a post-industrial society, but it is still an open question. Recall Lenski's five types of society. At first, there was the hunter-gatherer society, followed by horticulturalism--gathering and planting crops. Then followed agricultural, and industrial, and finally post-industrial.
During the industrial level of technological advancement, you start to see inequalities lessening. Life expectancy get longer, people start earning better wages for their jobs, etc. There is a lowering of social inequality during industrial societies, whereas hunter-gatherer societies, feudal societies, horticultural societies and agricultural societies are more caste systems and tradition-bound. Nobility had power over those who didn't, and you can see how the power structure is related to technological levels.
IN CONTEXT
There are some criticisms of the Kuznets curve theory. For instance, Canada is much more equal of a society than the United States is, yet the U.S. has approximately the exact same level of technological advancement.
Clearly there are extraneous variables influencing the level of inequality within the country, such as culture, political and social policy. It’s not possible to explain everything by technology, but it does provide a way to start thinking about societal change, technological advancement, and levels of inequality.
The current post-industrial society, with computer technology, has created an information processing class of people as opposed to manufacturing groups of people. Not everybody has made the leap into this information processing world, so you are seeing an emerging creative class of super-empowered individuals who are becoming far more powerful relative to the social inequality of people in the past. As mentioned, it's an open question as to where society might go in the future.
Source: This work is adapted from Sophia author Zach Lamb.