The phrase "in on" is a preposition that means to be involved in something or to have knowledge about something, especially a plan or secret. It's often used to indicate that someone is part of a group or knows about something that is happening.
Simple Example: "I was in on the surprise party for Mark."
Another Example: "She is in on the secret."
While "in on" primarily refers to being involved or knowledgeable, it can sometimes imply a level of exclusivity, suggesting that not everyone is aware of or part of the situation.