Sync / No Sync is an indication of the validity of the ladder display when on-line monitoring. If "Sync" is shown then the PLC is transferring all the polled information to the PC within one PLC scan. Therefore the information displayed in the monitored ladder view is accurate and 'in sync'.
If "No Sync" is shown then the monitored information is being transferred from the PLC to the PC in more than one PLC scan. Therefore the information shown cannot be relied on to be accurate as some data is from one PLC cycle, and other data is from a different PLC cycle.
If this occurs then the comms rate should be increased or the reason for the slow transfer investigated.
This is usually caused by too many "Ladder" view windows being open, and therefore monitored, at the same time.