Plain English Meaning
When you turn to or away from south while flying in the Northern Hemisphere, the magnetic compass spins faster than you are actually turning. So when rolling out on a southerly heading, you have to let the compass go past the heading you want before stopping the turn.
Full Definition
Errors shown by the magnetic compass when an aircraft turns to or from a southerly heading in the Northern Hemisphere. When turning from a southerly heading, the compass leads the actual turn — it shows a turn in the correct direction but at a faster rate than the aircraft is actually turning. To compensate, the pilot rolls out of the turn after passing the desired heading by an amount roughly equal to the latitude minus a small allowance for the bank angle. The cause is magnetic dip: the vertical component of Earth's magnetic field pulls the , and the resulting tilt produces predictable errors during turns through southerly .