If we throw a thumbtack onto a table once, we would have a hard time predicting whether the point would touch the surface of the table or not. But if we repeat this experiment many times, we will see that the number of times the point touches the surface divided by the number of throws will eventually stablize at a specific value.
Repeating a series of trials will produce similar, but not identical, results for each series. This observation is called statistical regularity.
The same idea occurs in games of chance, demographic statistics, quality control of a manufacturing process, and in many other parts of our lives.