The earth has only so much mineral resource. There is only so much time and human labor available. Individuals have only a finite amount of money to spend to satisfy their needs, wants, and desires. The fundamental problem of economics is that while resources are scarce, human wants, needs, and desires have no limits. We must forgo some desires in favor of others.
Opportunity cost is the cost of choosing one alternative over another.
It is the benefit forgone when we make a choice. You can't do both. If you choose to go to dinner, your opportunity cost is whatever benefit you might have gained by going to the ball game. This idea Of Opportunity cost helps to explain how we make choices. We act to minimize our opportunity costs, or to maximize our benefits by the choices we make.