Prosperity is a measure of how successful your villages are. The value of Prosperity is the sum of Food, Wealth, and Technology. Individual villages have prosperity, which can be viewed by selecting a village and finding the "Prosperity" label in the village panel. Your total prosperity is the sum of all your villages' prosperity and is shown in the blue circle in the upper-left corner of the screen. Some developments require an individual city to reach a certain prosperity by the end of the era, while others measure your total prosperity.