In the book "Macroeconomics" by Olivier Blanchard & David R. Johnson, the following appears: 

> In yet other countries, people who have emigrated to the United
> States bring home U.S. dollar bills; or tourists pay some transactions
> in dollars, and the bills stay in the country. This is, for example,
> the case for Mexico or Thailand.

>The fact that foreigners hold such a high proportion of
the dollar bills in circulation has two main macroeconomic
implications. First, **the rest of the world, by being willing
to hold U.S. currency, is making in effect an interest-free
loan to the United States of $500 billion**

By holding US bills of 500 billion, the rest of the world is making the US an interest free loan of $500 billion---could you please explain me how?