let someone in on something
phrasal verb with let verb uk/let/ us/let/ letting | let
向（某人）公开;允许（某人）参与（秘密的事情）
to tell someone about something that is secret, or to allow someone to become involved in something that only very few people are involved in
Debbie agreed to let me in on her plans. 黛比同意让我参加她的计划。