What does poorest country by population mean? I understand poorest countries based on GDP but what does poorest countries by population mean? Below is the link to the information I was looking at. 
http://worldpopulationreview.com/countries/poorest-countries-in-africa/
"Poorest country by population" doesn't really mean anything: I think this is just sloppy writing by the website. The table on the website is actually ranking countries by 

$$\text{GDP Per Capita}=\frac{\text{GDP}}{\text{population}}.$$

This is a measure of the value of goods or services that the average person in a country produces.