I believe that means when you are turning onto finals they do not want you to overshoot the extended centreline of the runway. You may drift too close to the other parallel runway centreline.
ie If 16L and 16R are the arrival runways. If the aircraft turning final on 16R drifts too much past the centreline he may encroach the extended centreline of 16L.
If they both do it at the same time then its not good at all.
I hope I have explained that correctly. The piloty types may correct me.
