let in on

US /let ɪn ɑːn/
UK /let ɪn ɑːn/
"let in on" picture
1.

让...知道, 透露给

to allow someone to share a secret or to know something that is not known by many people

:
They finally let me in on their plans for the surprise party.
他们终于让我知道了他们惊喜派对的计划。
She refused to let him in on the secret.
她拒绝让他知道这个秘密。