That seems very simple, but it has important implications. Resources must be consumed to produce the goods and services we all want. Resources are obviously finite. 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.