Imagine you have 26 population in a city.
You have at least eight grassland or flood plains tiles farmed next to a river, and you have already researched Civil Service (4 food per tile)
You then have 8 population working those tiles (4*8=32).
Each pop uses 2 food so 32-8*2=16
You have 16 surplus food, that means, you can support another 8 population, which can be working mines that yield 4 production, so you would now have 32 production, plus modifiers.
So basically, population generates production and science mostly, wealth if you want