Plain English Meaning
A setting where the heading indicator is left to spin on its own without being kept aligned to magnetic north automatically. The pilot has to set it by hand and check it from time to time because it will slowly drift off.
Full Definition
An operating mode of a remote indicating compass system (such as a slaved gyro/HSI) in which the directional gyro runs independently of the magnetic flux detector. Heading is no longer automatically corrected to magnetic north by the slaving circuit; the pilot must manually set and periodically reset the heading indicator to match the magnetic compass.