Because development encompasses many factors - economic, environmental, cultural, educational, and institutional - no single measure gives a complete picture. However, the total earnings of the residents of an economy, measured by its gross national income (GNI), is a good measure of its capacity to provide for the well-being of its people.
GNI, PPP (current international $)
This indicator provides values for gross national income (GNI. Formerly GNP) expressed in current international dollars converted by purchasing power parity (PPP) conversion factor. 
Gross national income is the sum of value added by all resident producers plus any product taxes (less subsidies) not included in the valuation of output plus net receipts of primary income (compensation of employees and property income) from abroad. PPP conversion factor is a spatial price deflator and currency converter that eliminates the effects of the differences in price levels between countries.

From July 2020, “GNI: linked series (current LCU)” [NY.GNP.MKTP.CN.AD] is used as underlying GNI in local currency unit so that it’s in line with time series of PPP conversion factors, which are extrapolated with linked deflators.
Typically, higher income countries have higher price levels, while lower income countries have lower price levels (Balassa-Samuelson effect). Market exchange rate-based cross-country comparisons of GDP at its expenditure components reflect both differences in economic outputs (volumes) and prices. Given the differences in price levels, the size of higher income countries is inflated, while the size of lower income countries is depressed in the comparison. PPP-based cross-country comparisons of GDP at its expenditure components only reflect differences in economic outputs (volume), as PPPs control for price level differences between the countries. Hence, the comparison reflects the real size of the countries.

For more information on underlying GNI in local currency, please refer to the metadata for "GNI (current LCU)" [NY.GNP.MKTP.CN]. For more information on underlying PPP conversion factor, please refer to the metadata for "PPP conversion factor, GDP (LCU per international $)" [PA.NUS.PPP]. 

For the concept and methodology of PPP, please refer to the International Comparison Program (ICP)’s website (https://www.worldbank.org/en/programs/icp).