Copperis obtained by building a Mine improvement on the resource. A mined Copper tile provides a total of +3 to the tile, an unmined Copper tile provides a +1 to the tile.
The resource (once the Bronze Working technology has been researched) provides the Bronze Weapons upgrade to all Melee units as well as some Mounted units (e.g. Chariot). Once Copper is connected, to gain the promotion, a valid unit must first visit a city connected to the Copper resource to gain the promotion.
Bronze Working also grants each city connected to Copper (whether from trade agreements with other players, or a mined Copper resource) +1 a turn.