The phrase you mention in your question does have meaning. But, it's the use of "must" that gives the phrase meaning. It's not an English idiom or anything. "You must know" is an example of using "must" to emphasize a verb. Example: "You *must* know something so simple!" I used the "*" symbol to show how that might be verbally emphasized. I don't think we have any expressions that hold no meaning at all. There are many English phrases that mean things other than they literally say, but not many I can think of that hold no significance.

You must know= you ought to know

If you have the full sentence some context would really help with this one.