The Aufbau principle states the way that electrons are filled in atomic orbitals. It dictates that electrons won't occupy high-energy levels at least the least-energy orbitals are filled.
In this case, in chromium, 3d orbitals get 1 electron before 4s get two electrons. In copper, each 3d orbital gets two elections before 4s gets two, which is a clear violation of the Aufbau principle.