Phrasal verb: Let (someone) in on (something) - Day 1

Hello. It's Day 1 of our 20-day phrasal verb challenge!

I'm going to let you in on a secret.

There are lots of phrasal verbs in English,

so you can't learn all of them quickly.

However, you can learn many of them

if you pace yourself.

Study a limited number of verbs at one time.

Be sure to learn them in a meaningful context.

Practice them well,

and make time to review.

What does this phrasal verb mean?

Has someone ever let you in on a big secret?

Were you able to keep the secret?