Standard deviation (is this what you mean?) is the average amount that a score differs from the mean. This tells us how much each score is different from the average score in a study. So the greater the deviation, the more spread out the scores are. One standard deviation on both sides includes 68% of the scores, while 2 standard deviations in both directions make up about 95% of scores. This applies to either statistics or research methods.
What is the James-Lange theory?