To pop, or popping, is the term used within Erfworld for the spontaneous appearance of things at the beginning of a turn.
Food pops at the beginning of each day/turn. Assuming the side can pay for the upkeep of its units, every unit will automatically pop enough food to nourish it for that day. Food appears already cooked and needs no preparation. It cannot be stored for later days, and no surplus will pop. It is unclear what happens if a side cannot (or will not) pay the necessary upkeep. Do the units begin to starve? Do they croak immediately? Do they disband, or become barbarians?
Units, including Royal and noble units, and presumably casters as well, all pop in cities after a certain period of time. Warlords are designated, or promoted, from regular units, and so presumably don't pop as such. Or perhaps it is only Chief Warlords that are designated, and regular warlords do pop?
The Ruler of each side apparently controls the popping orders of his city. It is unclear whether there is a queue and only one unit develops at a time, or if several (or all) available units are "growing" concurrently.