The expected value of an experiment is the probability-weighted average of all possible values. It is defined mathematically as the following:
$$E[X] = \sum_{x \in X}xP(x)$$The law of large numbers states that the average result from a series of trials will converge to the expected value. Roll the die to see convergence to its expected value.
Change the theoretical probability of the die to see how that changes the average and expected value.