Plain English Meaning
When you turn the airplane, the magnetic compass doesn't show the right heading right away. Heading north it reads behind where you actually are, and heading south it reads ahead. Pilots have to roll out of the turn early or late to end up on the heading they wanted.
Full Definition
The predictable inaccuracies displayed by a magnetic compass while an aircraft is turning, caused by the compass card's tendency to dip and lag or lead the actual heading because of magnetic dip and the aircraft's bank angle. In the Northern Hemisphere, turns to the north cause the compass to lag behind the true heading, and turns to the south cause it to lead ahead of the true heading. These errors are most pronounced near headings of north and south and disappear near east and west.