Your mic has latency — a small delay between when you sing and what gets recorded. The Sync slider compensates for that delay so your voice lines up with the beat to the millisecond.
Voice lags (→ move to +ms)
Beat
Your voice
+80 ms or more
Perfect — in sync
Beat
Your voice
0 ms / neutral
Record, listen with Play, and adjust if you hear the offset
Headphones remove echo and make the offset easier to hear
Every device is different — your ideal value may be 50–200ms
The value is saved automatically for your next session