What does the idiomatic expression in mean?

Please provide the full idiomatic expression you're asking about. "In" by itself isn't an idiom. For example, you might be thinking of:

* "In the know": Meaning to be aware of something that is secret or not generally known.

* "In the loop": Meaning to be kept informed about something.

* "In the bag": Meaning something is certain to happen or be achieved.

Let me know the full expression, and I can explain its meaning!