A medium of exchange is anything people agree to accept as payment.
Why it sounds harder than it is
It is three words for the middle step.
Without one, you would have to find someone who wants what you have and has what you want, at the same moment, in the same place. That almost never happens.
So a thing goes in the middle. You swap your work for the thing everyone accepts, then swap the thing for what you need. The medium is the thing in the middle. Economists needed a name for it and picked a formal one.
The phrase describes the object doing the work. It says nothing about you.
A concrete Australian example
Your pay lands on a Thursday. You have almost certainly never been paid in whatever your employer sells.
You were paid in dollars, and the dollars go in the middle. They buy groceries from someone who does not want your work, and they cover rent for someone who has no use for it either.
When you tap a card at the checkout, the reason it works is that the person on the other side is confident the next person will take those dollars too. Nothing else is holding it up.
What it is not
- Not the same as money. Money does three jobs and this is one of them. The other two are measuring prices and holding value over time
- Not a store of value. Something can be accepted everywhere today and buy less next year
- Not proof the thing has worth of its own. Acceptance is the whole qualification
Related
The last three things you paid for all used the same middle step, and none of them required you to know it had a name.