What does it mean when the SYNC button is blinking while I'm using rekordbox?

When the SYNC button is lit, both the beat position and BPM of the track are synchronized. When it's blinking, only the BPM is synchronized and the beat position is out of sync.

If you use pitch bend on a deck that isn't the master, the button will switch from lit to blinking. To switch back to lit (i.e. start syncing beat positions), press the SYNC button twice to turn off and turn Sync on again.