Earnings measure the effective incomes of people in a nation, i.e. just how much a "poor/middle/high income" in the country is worth. There are three kinds of earnings; Poor Earnings, Middle Earnings, and High Earnings.
Earnings affect the social mobility of a nation. High levels of poor earnings means that more Poor people can save up and graduate to become Middle Income earners and high levels of middle earnings means that more middle income earners can go on to become Wealthy. The opposite of this is also true; low levels of high earnings will cause wealthy people to fall back into the middle class and low levels of middle earnings will cause middle income earners to become poor.