The long-run average result of something you repeat, like opening booster packs. You get it by multiplying each outcome's value by its chance and adding the results, so a pack's expected value is the average card value per pack over many packs.
For new collectors
Expected value describes the average over many packs, not what any one pack holds. It's built from market prices that change daily, so an expected value figure is only as current as the prices behind it.