In order to answer this question, I’m going to quickly summarise the history of money;
In order to further enrich our lives, people began to trade. If somebody had , say, 2 goats and another person had 2 chickens, it would make sense for them to trade a goat for a chicken. One of the problems that sprung up from this ‘free trade’ agreement between two parties, was that somebody may want a chicken, but the owner of the chickens doesn’t want a goat. To this end, an intermediary (or medium) was needed. It was (and still is) called money. It was observed that some products were used mainly to facilitate transactions but had no other real use than that. Historically this has been gold for large transactions and silver for smaller ones.