Population is the largest contributor to Raw Production (which can also be increased by asteroid mining, and precursor planet boni). As above, Raw converts into Income, Research, Planetary and Ship Production. Economic starbases add a percent to Raw Production, as do some governmental technologies.
I tend to prioritize population growth and pop caps in my research/planetary builds, as the faster Raw Production goes up, the faster everything else does. Balancing in happiness is always required, as is influence/tourism, diplomacy, and fleets.