Soheil is correct, but it can also be used informally to mean 'although'. We could go out on Friday, mind you, it is supposed to rain. Bob's always late! Mind you traffic is always bad at this time.

"Mind you" is usually used for warning a person. Here are some examples: 1. You need to stop acting like a child. You're not in primary school, mind you! (In this sentence, you use "mind you" in order to warn the person who is acting like a child that he/she needs to stop acting like a child because he/she is not a child anymore.) 2. Stop wasting your money like that! You just lost your job, mind you!