The answer is gold. Your $20 is worth a certain amount of gold, and that is what the government owes you. $20 in gold. These days you'd be hard pressed to get the government to give you gold, of course, but that is the answer to your question.

As an example, British pound notes have the wording "I promise to pay the bearer on demand the sum of five [ten/twenty/fifty] pounds", which dates back to a time when you could actually exchange the notes themselves for gold.