Many biological variables such as height vary similarly: a few individuals have the smallest values (are very short), a few are at the higher end, but most are in the middle. A graph of the numbers against the value usually results in a bell-shaped curve, which is known as the Normal Distribution.
The mathematics of the Normal Distribution look complex, but the results can be simulated by throwing a number of dice.
If 2 dice are thrown the most common total will be 7, because many combinations from the individual dice total 7. For example:
|Die 1||Die 2|
Whereas only one combination totals 2, and only one totals 12.
Similarly, if you throw 6 dice you have a much greater probability of obtaining a score of 21 than 6 or 36. This web-page simulates the throwing of 6 dice 500 times. It plots the number of times each of the scores from 6 to 36 is obtained. You will see that the graph resembles the normal distribution.