From SKYbrary Wiki
A flight director is a guidance aid that is overlaid on the Attitude indicator and shows the pilot of an aircraft the attitude required to follow a certain trajectory.
The flight director computes and displays the proper pitch and bank angles required in order for the aircraft to follow a selected path. Flight director guidance can be used in both manual flight and with the Autopilot engaged.
The FD is generally used in direct connection with the autopilot, where the FD commands the AP to put the aircraft in the attitude necessary to follow a trajectory. The FD/AP combination is typically used in autopilot coupled instrument approaches such as CAT II and CAT III Instrument Landing System (ILS) instrument approaches.
The exact form of the flight director's display varies with the instrument type.