The rule that confirms that as the number of trials increases, the actual probability approaches the expected probability. If you flip a coin 1000 times, you might get 470 heads. Or 550 -- expected probabilities of .47 and .55, respectively. But, damn it, the limiting value of that probability as the number of flips increases without bound is going to be .5.
Brother, if you continue playing that slot machine, the law of large numbers is going to get your ass.