It sounds more complicated than it is. Basically, MVRV is the ratio of Market value to Realized value. Realized value being the sum of the price when each Bitcoin was moved last, and market value being the full market cap of outstanding Bitcoin. The mean of this ratio is about 1.6, just historically speaking. Adding a Z-score into it just looks at where we are in terms of standard deviations from this mean. A Z-score of 1 means we are 1 standard deviation away from this mean, while 0 indicates that we are at that mean presently. And negative Z-scores of course mean we're below the mean. It's the yellow line on this graph: https://charts.bitbo.io/mvrv-z-score/