The Law of Averages is the belief that a particular outcome of a random event will occur at a frequency that is similar to its probability if the random event happens many, many times.
For example, the Law of Averages says that if you roll a die 100 times, you will roll a 4 approximately 16% of the time (which corresponds to the 1/6, or .16, probability of rolling any number once).
Honestly, it should probably be called the Law of Probabilities.