What is rmssd in simple terms and what is meant by threshold normal values?what are the normal values ?is over , she >0.76 rmssd 0.13 significant?

It's not that simple: The Root Mean Square of the Successive Differences (RMSSD) is a time domain used by electrophysiologists to asses heart rate variability. Normal values depend upon the study and time duration involved and the standard deviation f the test. These numbers need to be assessed in context, preferably by a cardiologist familiar with your condition.