Money or currency is a way to represent value

It is a medium of exchange

Could your boss pay you in cows? Could you take those cows and trade them for a car? Yes but this would be difficult.

So if your boss gave you a cow voucher “money” and you could trade that for a car, then you wouldn’t have to lug the cows around.

It facilitates trade

Why a US dollar, or another currency, has value is a whole other question