Simply put, it's what you give up in order to do something. Imagine, for example, that you dream of becoming an engineer or a chef. If you opt to become a chef, you give up the experience of being an engineer and all that goes with it. That's an opportunity cost of becoming a chef.
Opportunity cost is also often defined, more specifically, as the highest-value opportunity forgone. So let's say you could have become a brain surgeon, earning $250,000 per year, instead of a chef earning $50,000. In that scenario, your opportunity cost, salary-wise, is $200,000. (Of course, you should also consider factors such as your enjoyment of your chosen profession.)
Read more on Opportunity Costs.