Population development is crucial for city development, resource production and military development of the population. Gold is aquired via tax. In the right column of the game interface, you can see two population-related values: the City Population and Idle Population.
City Population: total population of all residents in all residential buildings in the city. You can increase city population by building residential areas.
Idle Population: total population of all residents that are idle. If Idle population is more than 25%, the population's loyalty will drop, in effect, lowering the yield of the city resources.
Food Consumption: the population and armed forces in the barracks every day will consume food, Loyalty will drop if there is a lack of it.
Loyalty: All city productions will be multiplied by the percentage of Loyalty, including gold production.
Charity: If Loyalty declines you donate grain to the common people via Charity. Fore every 1 grain donated Loyalty will increase.