Please explain what the average mean is

Lets say that for every city the height of the population is measured. Every city would have a different mean, and the average mean would be the average of the means of each city. In statistics, every trial has a mean, and the average mean would be the average of the means over each trial. The average mean is almost always normally distributed, and has properties which depend on the number of trials. Here, each trial is the height survey of each city, and the average mean would be the national average of height, which is measured by taking the average of all the height means of each city.