Production in Sid Meier's Civilization is represented by and referred to as Shields. Production is used to produce units and buildings- the more production in a city, the faster a unit or building is constructed.
Under some governments, units require a certain amount of production each turn to stay alive. This is marked by placing a single shield to the left of the 'free' production, per shield consumed by unit. More units produced by a city require more production to stay alive. Production used to maintain units is not used when producing more units and buildings.
Production is produced when a citizen works a plains, mountain, forest, resourced grassland, swamp (with oil), or hill (with mines) tile. This can be increased by building mines (if possible on the terrain type) or railroads on the worked tiles, or building factories and Mfg. Plants, and nuclear, hydro, or power plants to further increase production.