After Sunrise Economy
Population is dudes. 1 Population consumes 1 Food and outputs 1 Production each turn. Unfed Population units have a chance of dying each turn.
Industry is factories and similar infrastructure. An active unit of Industry outputs 2 Production and consumes 1 Fuel each turn. To remain active a unit of Industry must be staffed by a unit of Population. 1 Population cannot staff more than 1 Industry.
Production is an abstract measure of your potential output of stuff like tanks, bombs, machine tools, bridges, shelters, mod Toyota technicals, and so on. Production cannot be stockpiled, so if it is not turned into something in a turn, it is wasted.
Food is food and comes from farms, fisheries, flocks and all the rest. 1 point of food production produces X Food each turn. Food can be stockpiled.
Fuel comes from coal mines, oilfields, natural gas pockets, and so on. 1 point of fuel production produces X Fuel each turn. Fuel can be stockpiled. Nations without the Black Gold advantage are limited in the amount of fuel production they can buy at nation creation.