Definition
to hand a criminal over to law enforcement, or to report yourself to them for committing a crime
向警方自首；告發
She decided to turn in the criminal she witnessed.
她決定將她目睹的罪犯交給警方。
If you know he committed a crime, you should turn him in to the authorities.
如果你知道他犯了罪，你應該把他交給當局。
He ultimately chose to turn himself in to the police.
他最終選擇向警方自首。