Like if you wanted a news paper you would have to give the news man a dollar, But if that news paper was scarce it would be 2 dollars because the demand of how many people want it. Your opportunity cost would be your opportunity to buy the local 50 cent version.
Economics is kinda like the process to get what you want.
This is what you use to get the things you want. You essentially get money for your time, like working or doing a service.
A "dollar" is what we use in Americas mixed economy other places have different currency like pesos, or yen. A mixed economy is free to anyone, but with government regulations. Almost a "Market economy" where there is no government regulations.
Trading is giving something the other person wants, for something you want.
In Jail instead of money inmates will trade fro what they want.
This is often seen in "Traditional economy's", poor places, old places. Other country's have jail like systems called "command economy's" that are all government controlled.