Z-scores are a stand-in for the actual measurement, and they represent the distance of a value from the mean measured in standard deviations. So a z-score of means the measurement is 2 standard deviations away from the mean.
The Empirical Rule When a population has a histogram that is approximately bell-shaped, then Approximately 68% of the data will be within one standard deviation of the mean. In other words, approximately 68% of the data will be between µ − σ and µ + σ. Approximately 95% of the data will be within two standard deviations of the mean. In other words, approximately 95% of the data will be between µ − 2σ and µ + 2σ....William Navidi, Barry Monk Elementary STATISTICS Second Edition
Figure Relative units of Spectral Power Density (vertical axis) for photon emissions for the amplitude enhanced at the 22 Hz band as a function of the various combinations of malignant and non-malignant cells. Vertical lines refer to standard deviations.