If you hover your mouse over 'Z-Score' on your account(s) page(s), it explains the use of it with regards to MyFxBook. Here it is strictly used to estimate the probability of a win followed by a loss, or a loss by a win. A stable number means you have a good, consistent trading system. You don't want 'randomness' in a system.

Z-score is a principle of statistics-- when I took my college statistics class, this was a primary topic. Basically, z-score is a reading of the distribution of data for probability in a statistical sense. The z-score is calculated by taking a data point and subtracting the distribution mean and dividing the total by the standard deviation. A positive number means that the data is higher than the mean, and a negative number means the data is below the mean. The best example of this is a class with several grades. Each grade is assigned a z-score according to the distribution mean and standard deviation. The higher the z-score, the higher the variation of that student's grade from the mean.

There are other methods by which probability can determine if your next trade will be a winner or loser, but Z-score is a simple calculation that takes into account all your current numerical data. I'm really not the most qualified person for a detailed explanation-- I barely passed my Stats class and only made it because my project was pretty good (determine differences in gasoline prices in various regions of the city).