Population is another major part of your faction, each planet has a certain amount of population assigned to it, that amount is the maximum number of buildings on each planet times 200, so if your maximum number of buildings on the planet is 100, then the maximum population is 100*200=20,000. Population is very useful to make your upkeep smaller as for every 350 people that live in your faction your total up keep is reduced by 1. This may not seem like a lot but it is when you have a lot of people.
Population grows at 2% per week if you don't have any bonuses. To calculate how much it grows with a bonus multiply that bonus by 0.02 and then add 2. (Ex. for 10% - 0.02*10+2=2.20% per tick)