HDI, or human development index is a tool developed by the United Nations to measure and rank countries' levels of social and economic development based on four criteria: life expectancy at birth, mean years of schooling, expected years of schooling, and gross national income per capita. The HDI makes it possible to track changes in development levels over time and to compare development levels in different countries.
HDI is commonly used to measure development of countries. Lets take two countries and compare them. Sierra Leone has a really low HDI rank comparing it to, for example, Cape Verde. Cape Verde has almost three times the life expectancy than Sierra Leone, twice the education index, but almost the same GDP per capita as Sierra Leone. Which is justified by the size of the population of Cape Verde compared to Sierra Leone.