Observe a random variable X very many times. In the long run, the proportion of outcomes taking any value gets close to the probability of that value. The Law of Large Numbers says that the average of the observed values gets close to the mean X of X.
The random variable X is the total number of spots on the up faces of one or more dice. Choose the number of dice by clicking Fewer dice or More dice. To see the mean number of spots X, click Show mean. Fill in the number of rolls and click Roll dice. The graph shows the average of the spots for all of your rolls. As you make more and more rolls, eventually gets close to X.