What is supraventricular ectopic activity mean? Also what is n-n interval?

Heart beat starts. In an odd place in the small chamber(s) of the heart instead of the node that is supposed to fire off the heart. This electrical trigger is above the level of the big chambers (the ventricles) so it is supra ventricular and it's not supposed to work that way so it's ectopic too.The n-n interval is a distance on the ekg that shows the length of 1 entire heart beat. Search 'heart rate variability'.