Plain English Meaning
The needle on the navigation instrument is showing how far off course you are, and which direction to fly to get back on course. A centered needle means you are on course; a needle to one side means the course is over there.
Full Definition
The movement of the course-deviation needle on a navigation indicator (such as a VOR or localizer display) away from its centered position, showing that the aircraft is laterally displaced from the selected course. The direction the needle moves indicates which way the course lies relative to the aircraft, and the distance it moves indicates how far off course the aircraft is.
Context Anchor
Seen while using cockpit to follow a path, especially during in .