In Civilization 5 Population , is a term used to represent the Citizens living in each of the population centers, or cities, of your civilization. They are the main workforce of the Empire, occupying all the labor slots in and around your cities. As such, they're paramount for the productivity of each city, and the Empire as a whole.
Population is represented by a number used as a marker for both the amount of Citizens and general size of a city. The number is not literal, it is instead used for gameplay purposes (for example, each single Citizen can occupy a single job slot). If a City has 1 population, or Citizen, it does not mean there is literally only one person living there - it means the city is a very small village. If you have 10 Citizens, your city is a decent, sprawling population center. If it is 20 or more, you have a metropolitan area, covering even nearby tiles.
The Population as a measure of city size is also represented by the graphic size of the city itself. As mentioned above, a newly-born village will barely occupy the middle of its tile, while a sprawling metropolis will spill into neighbor tiles as well.
Each new city starts with a Population of 1, or 1 Citizen. It can subsequently grow its Population, or shrink it, according to the rules of Food consumption and distribution. However, a city always has at least 1 Citizen. For more info on Population growth, check the Food article.