Okay, here's a scenario: there's a doomsday event, the world is on the brink of extinction, and only a few people remain on a desert island with limited resources. 

NB: Before the extinction level event, you had a lot of money in your bank account; you even had some notes in your pocket. 

Let's say there’s someone who grows pineapples on the island. 

Will the numbers in your bank account or money in your pocket have any meaning to the person you want to receive pineapples from? 

What use are your notes and coins to them? 

The person growing pineapples would give zero fucks about this thing you call money. 

In exchange for pineapples, they would want something of value, a good or service that they could use on that island. 

So, if you knew how to find coconuts and make clothing from the husks and the pineapple seller needed clothes, then we’re talking. 

You both have a product that you can exchange with each other. 

However, it might not necessarily be the case that you would have someone who has the product of clothes ready at the same time as the pineapples are ripe and suitable for sale. 

What if they each want what the other has at the same time and place? 

This situation is called the double coincidence of wants. 

You both have something of value and want something you don't have simultaneously. So, how can you trade with each other?

You may strike a deal to exchange a certain amount of pineapples for some clothes. 

You may make a contract and have many trading partners on the island, exchanging goods and services for agreed amounts at varying times. 

In such a scenario, this would create a barter economy.