Definition
to hand a criminal over to law enforcement, or to report yourself to them for committing a crime
向警方自首；告发
She decided to turn in the criminal she witnessed.
她决定将她目睹的罪犯交给警方。
If you know he committed a crime, you should turn him in to the authorities.
如果你知道他犯了罪，你应该把他交给当局。
He ultimately chose to turn himself in to the police.
他最终选择向警方自首。