z score is a measurement of how many standard deviations above the mean or below the mean.
for example in the below image the value 2.9 is 2 standard deviations away from the mean of 2.5, so 2.9 has a z-score of 2.
3.1 has a z-score of 3.
Z-score formula
z = (x – μ) / σ
Z = standard score
x = observed value
μ = mean of the sample
σ = standard deviation of the sample