A z-score is a statistical measurement that describes how far a data point is from the mean of a dataset, expressed in terms of standard deviations. A z-score of 0 indicates that the data point is exactly at the mean, while a positive z-score shows that it is above the mean, and a negative z-score indicates it is below the mean. Z-scores are useful for comparing different datasets or understanding the relative position of a value within a distribution. They help identify outliers and assess probabilities in a standard normal distribution, which has a mean of 0 and a standard deviation of 1.