See the current, or flow.
Anything that assist the flow of value. Examples: money, gift cards, checks, IOUs, bicycles. busses. Using them gets someone or something to the next useful place or situation.
A system of money that is used in a particular place at a particular time.
A potential problem with currency is that there may be too little of it or too much of it.
When there is too little of it people cannot purchase what they need--goods, service, labor, capital.
When there is too much of it it is relatively worthless. Explain this.
So a critical need is for currency to be available in just the right amounts. This can be tricky and is open to manipulation by the people able to create or destroy currencies.