The term “sync” refers to the fourth cable in RGB setups: The “s” in RGB. This fourth signal coordinates the horizontal and vertical synchronization of color and brightness information sent separately over the three RGB lines.
If you purchase RGB cables, you’ll often be able to choose what signal should be connected to the “sync” line. Video game consoles can safely use both Composite Video (CVBS) and Luma (S-Video) as sync signals with RGB SCART. Some consoles even generate their own sync signal, but that doesn’t often provide any benefit.
The screenshot below shows what happens when a sync signal is disconnected. Since the screen is static, the issue is mild, but if a game were being played, the signal would go crazy.