Frankly means truthfully. Example: Can I speak to you frankly about a colleague? A very famous line from the film Gone With the Wind was said at the end by Rhett Butler to Scarlet O'Hara" : "Frankly my dear, I don't give a damn".

Frankly, you need to be very careful how you use "frankly". To be frank means to be honest even to the point where it would make the other person uncomfortable. So, when you say the word "Frankly" you are essentially warning the listener that you are going to be saying some hard truths. And, frankly, that's the way it should be!

To be frank is to be (brutally) honest. Perhaps you're about to say something that the listener might not like... but you feel you must say it anyway. Song time! This one has the lyrics: http://www.youtube.com/watch?v=h9l_zAypP7Q