A city requires 2 Food per Citizen (another term for " Population") per turn to avoid starvation. A city acquires Food (as well as other yields) by assigning its Citizens to "work" the land around the city. The city can work any tile within 3 spaces of the city that is also within the civilization's borders, provided as well that it is not being worked by another city. Cities cannot work tiles occupied by wonders.
Left to its own devices, the city will assign as many Citizens as needed to acquire its Food.