What Is Standard Deviation?
Standard deviation measures how far, on average, the values in a data set sit from the mean. A small standard deviation means the numbers cluster tightly around the average; a large one means they're spread out. It's the most common way statisticians describe variability, from test scores to manufacturing tolerances.
Example
Two classes can both average 80 on a test, but one class has scores tightly bunched near 80 while the other ranges from 40 to 100. The second class has a much larger standard deviation.