That would be the rate of the ventricles. The atrium might flutter at 300 BPM, but the lower number is the beats of the ventricles.

Flutter is quite easily treated via a catheter ablation. You should be referred to an electrophysiologist and see about an ablation if the flutter is not otherwise controllable. You had a successful cardioversion, and that's great. If the flutter returns, especially in the next few weeks, another cardioversion might still work....probably will. However, the frequency may rise, in which case you will need an ablation. Day surgery, no big deal, and it is 95% successful in the right hands.