What Is Z-Score?
A z-score converts a raw value into a standardized measure of how far it sits from the mean, expressed in standard deviations. A z-score of 0 means the value equals the mean; positive means above it, negative means below it. Z-scores make it possible to compare values from different data sets that use different scales.
Example
A test has a mean of 70 and a standard deviation of 10. A score of 85 has a z-score of (85−70)/10 = 1.5, meaning it's 1.5 standard deviations above average.