In probability theory, the expected value of a random variable is the long-run average value of repetitions of the experiment it represents.
For example, if you are rolling one die, the odds are 1 in 6 that you will roll a 1. Mathematically this computes to a 15% chance of rolling a 1. Expanding this to 100 throws, you would normally expect to roll a 1 approximately 15 times. This is the expected value of rolling a 1 one on a die.