Plain English Meaning
When you turn the airplane near a heading of north or south, the magnetic compass doesn't show the right heading. Near north it reacts slowly and shows a heading behind where you actually are. Near south it reacts too quickly and shows a heading ahead of where you actually are.
Full Definition
Errors in the magnetic compass indication that occur during turns to or from northerly or southerly headings, caused by the vertical component of the Earth's magnetic field acting on the tilted compass card. When turning through north in the Northern Hemisphere, the compass lags behind the actual heading; when turning through south, it leads the actual heading. The size of the error roughly equals the latitude.
Context Anchor
Seen when using the magnetic compass during instrument flying, partial-panel flying, or any situation where the compass is being used to confirm heading during a turn.