Standard deviation is a measure of how spread out a set of numbers is around their average, with a larger standard deviation meaning the values vary more.
If most adults score close to the average on a questionnaire, the standard deviation is small. If scores range widely, it is large. It is calculated as the square root of the variance.
Standard deviations are used to compare results. On many standardised tests, a score of 100 is average and one standard deviation is 15 points, so about two-thirds of people score between 85 and 115.
Some ADHD and autism research uses standard deviations to describe uneven profiles, where some abilities are well above average and others well below. That unevenness is common in neurodivergent adults. It's worth describing rather than averaging away.