Plain English Meaning
The line on the navigation display that tells you whether you are on the path you chose. If it sits in the middle, you are on the path. If it leans left or right, you have drifted that way and need to turn toward it to get back.
Full Definition
The vertical pointer on a Course Deviation Indicator that shows the aircraft's lateral position relative to the selected course. When the needle is centered, the aircraft is on course. When it deflects left or right, the aircraft is off course in that direction, and the pilot must steer toward the needle to return to the selected track.
Context Anchor
Seen on navigation instruments during navigation and when a selected course.