Can you describe a heart flutter?

Yes. Palpitations refer to a fluttering sensation in the chest which most commonly is related to the heart beating fast and/or more forcefully and/or beating irregularly.
Yes. The most typical pattern is that the atria contract at 300 times a minutes and conduct to the ventricles in a 2:1 pattern causing a heart rate of 150. There are variations of this, of course.