Population means the number of people in a geographic area. It can also be used for subgroups of people or animals. The city's population has passed 8 million––who knows what that means for the rat population living off their garbage? Ugh.
The word population––and also the word populace––derive from the Latin populus, "people." To remember that population is connected to people, think about the words popular, populist, pop culture, pop music. Don't think about pop corn––unless you're thinking about the movie-going population that lives on it.