A monopoly is exclusive control over a particular resource.
A copyright in a particular work can usefully be conceived of as a monopoly over that work and its uses, albeit for a limited time.
Economic theory typically sees most monopolies as inefficient uses of resources. These inefficiencies are harm to the public good. This harm is justified in copyright law by claiming that the incentives a monopoly provides to would-be creators balance it out. However, this view of things is being challenged more and more in recent years by critics who feel that copyright terms are too long, or that creators have motivations other than monetary reward.