Your city will grow, or add more Citizens, with surplus Food. Any amount of Food over the total consumption of a city's Citizens, 2 per, is considered surplus, and is added as progress towards a new Citizen.
A city will add all surplus Food towards growth if its current Population is 2 or more below the city's Housing limit. If the city is 1 Population below its Housing limit, its growth will slow to 50%. When the Population is at the Housing limit, it will grow only at 25% the normal rate. A city can grow to 5 Citizens over its Housing limit at this rate, and then it will stop growing entirely.
A city will stagnate and not grow if it does not have surplus Food. If it makes less Food than its Citizens consume, the city will starve and slowly lose Population until it is able to support itself again.