Where it comes from
Copper hides in the rock from a little below the surface down. Look for green verdigris staining on stone, then dig toward it with a pick. See Finding ore and Mining and mauls for how veins are read.
Hollows also leave copper scrap, which smelts the same way. See Copper scrap.
What it is for
Nuggets are smelted into copper ingots at the Smelter. Building the smelter itself takes a handful of them.