Opportunity cost is the value of the best alternative you gave up in order to take the option you chose.
It reframes decisions that look free. Money kept in a low-yield account has an opportunity cost equal to what it could have earned elsewhere; an hour spent on a task has the cost of what that hour could otherwise have produced. The idea is most useful for choices that feel costless precisely because no money changes hands — which is where the largest unnoticed costs usually sit.