Mistborn is a fantasy book about people with magical abilities called allomancers.
Allomancers "burn" (metabolize metals in the body for magical powers) ingested metals, thereby enhancing various physical and mental capacities.
A copper allomancer is referred to as a coppercloud. An Allomancer burning copper is able to hide themselves and others from "Seekers" by hiding Allomantic vibrations caused by burning metals. The radius of effect around an Allomancer burning copper is known as a "coppercloud" (Hence the name copper Mistings go by).