As your city grows, so does its population of peasants. You can tap this to find out how many are idle and the rate your population is growing by.
Idle population are the peasants that can be used to construct and upgrade most buildings and can be trained as troops.
As you construct and upgrade buildings, some of the idle population becomes employed, permanently increasing your total population.
Any idle population trained as troops are moved out of both the total and idle population pool, making room for more to join.

TIP: If you neglect housing you will run out of idle population (all of your peasants will be employed in the buildings you have made) and you will be unable to train new troops or upgrade buildings. You need to keep building and upgrading housing to increase your total population capacity

TIP: Building and upgrading housing also increases your population growth rate, as does your overall population morale