When you let someone in on something, you let that person know what's on your mind. This can be anything: a secret, a future plan, your school project, ... You're telling someone a piece of information that is usually not widely known. For example you can tell your friend, "I'll let you in on our plans for the summer, but don't tell anyone because they're not final yet." You can also say, "Don't let anyone in on this problem before we find a solution." This means don't tell anybody until we figure out what to do with the problem.
A: We might go to the Florida Keys before the end of the summer.
B: Really? That's awesome!
A: Yeah. My parents have been keeping it a secret, but my dad let me in on it last night.
B: Why have they been keeping it a secret?
A: Because we're in summer school, and we might lose concentration on our classes.
B: Oh. I guess that makes sense. You must be excited.
A: I am. It's actually a surprise for my sister's birthday. So my dad told me not to let her in on the secret. So don't say anything.
B: Don't worry. My lips are sealed.
My lips are sealed. - I won't reveal the secret.
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