Sync And Unsynced

[PK]

Hey all again, 
Recently I've watched the EFGT Finals (PtRvY vs ICy), at Petar's entry the description says that he hates when Youtube unsync's his videos. Can anyone kindly please tell me what does that mean ?

Thank You 

[value]

editing is synced to the beat
youtube makes it unsync
=
effect may pop up too late (unsync)