The population density is the quantification of the amount of people who live in a certain area. In units, it is the quantity of people per squared kilometre. Therefore, densily populated areas are those which have more people per km2.
There are two types of factors which determine the population density in an area:
Physical factors: like climate, available resources, or the relief.
Human factors: political and social conditions
If focusing on the factors abovementioned in the question:
Climate: areas with warm weather usually concentrate the largest world population shares. For example, it is possible to grow crops there. This is not the case in other places with extreme weather such as the North Pole or the Sahara Desert.
Resources: areas with resource availability such as energy sources (coal, oil), and other types of raw materials are more densely populated (US, Western Europe).
