let in on
US /let ɪn ɑːn/
UK /let ɪn ɑːn/

1.
einweihen in, teilhaben lassen an
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.
Sie ließen mich endlich an ihren Plänen für die Überraschungsparty teilhaben.
•
She refused to let him in on the secret.
Sie weigerte sich, ihn in das Geheimnis einzuweihen.