A z-score represents the amount of standard deviations a value is away from the mean average. Z-scores are calculated with a probability formula that requires the standard deviation, the population mean and the element value.
A z-score of less than zero is a value below the mean, while a z-score of more than zero is a value above the mean.
A z-score can be converted into a value that represents the probability of something occurring. For example, a z-score could determine the probability of people with a specific shoe size walking into a shoe store given that the amount of shoes per size was equally distributed.