The z-score is a measure of how many standard deviations a particular data point is from the mean of a distribution. A positive z-score indicates that the data point is above the mean, while a negative z-score indicates that it is below the mean. The magnitude of the z-score gives an indication of how far the data point deviates from the mean. It is calculated by subtracting the mean of the distribution m from the individual data point X and the difference divided by the standard deviation of the distribution s.

z=\frac{X-m}{s}

In R, you can use the scale() function.