To make it sweet and simple, opportunity cost is what you've given up to buy something that you really wanted. 

For example, a pair of heels and a pair of earrings. 

While both are super cute, you gravitate more towards the heels and decide to purchase them, leaving the earrings behind. 

Every choice that you make has trade-offs, and when choosing one thing over another you'll miss out on what that thing had to offer.