A slaved compass system, when installed in an aircraft, is continuously corrected by the remotely connected magnetic direction sensing device (usually a flux valve). An un-slaved system is corrected manually by using a knob on the panel mounted unit to set the compass card.
See example images below. The NSD360 is an un-slaved compass system; it has a knob on the bottom right side to set the compass card. With this particular model, the knob is a push button that needs to pushed & turned at the same time to set the card. On the other hand, the KI-525A is a slaved compass system without a manual compass card set button.