True.
Monetary history shows us that at first the exchanges that were made by barter were replaced by precious metals. Over time, there was the introduction of paper money, which should necessarily be backed by some metal with government guarantee. If it were necessary, the paper money would be exchanged for the metal in it.
Nowadays, the concept of fiduciaridade of the currency has been agreed. This means that there is enough confidence in the Government that there is no need for mandatory conversion. Thus, the currency is backed by the trust that citizens have with their government.