Population is a term used in Civilization V to represent the Citizens living in each of the population centers, or cities, of your civilization. Citizens 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 that indicates both the number of Citizens inhabiting a city and its general size. This number is not literal, and is instead used for gameplay purposes (for example, each individual 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. A city with a Population of 10 is a concentrated urban area, and a city with a Population of 20 or more is a major metropolitan area. The size differences between cities are represented graphically as well as by Population - whereas a newly founded village will barely occupy the middle of its tile, a sprawling metropolis will spill over into neighboring tiles.
Each new city starts with a Population of 1, or 1 Citizen. It can subsequently grow or shrink its Population according to the rules of Food consumption and distribution, though it will always have at least 1 Citizen.