Imagine a standard dice. That’s a six-sided cube, no Dungeons and Dragons version here. If you cast it a single time, what value would you expect to roll?
Simple probability tells you that each side has a 1/6 chance of landing up, and you can find the expected value by multiplying each of the six possible values against their probability, and adding those products: (1x0.1667) + (2x0.1667), and so on and so on. The final sum would be 3.5, or, the expected value of rolling a fair standard dice one time is 3.5.
Of course, you can’t actually roll 3.