Plain English Meaning
When you turn through south in the Northern Hemisphere, the magnetic compass shows the turn happening faster than it really is. To stop on the heading you want, you have to keep turning past it before rolling out.
Full Definition
A magnetic compass error in which the compass card lags behind, or shows a slower change than, the actual heading change of the aircraft during a turn. In the Northern Hemisphere, lagging error appears during turns through a southerly heading, where the compass momentarily indicates a turn faster than the aircraft is actually turning, requiring the pilot to roll out past the desired heading. The error is caused by the vertical component of Earth's magnetic field acting on the compass card as the aircraft banks.
Context Anchor
Seen in magnetic compass discussions, especially when learning why a compass behaves oddly during turns near north and south .