Throughout the history of American
paper money (
currency) different types have been issued. Some currency notes were issued with silver backing them up--Silver Certificates. With Silver Certificates technically a person (prior to 1964, after 1964 this law was changed and this no longer applies)could go to a Federal Bank and redeem the Silver Certificate for a like amount in silver dollars. Again, this is information available online at places like Wikipedia.
There were also Gold Certificates that prior to 1934 could be redeemed for gold coins. See, at one time our money was backed by REAL gold and silver. It no longer is. Now it is "backed" or "supported" by good faith
only.
OK, so along come United States Notes which were actually issued by the US Government as general circulation currency. They were as good as any other currency and are still perfectly good to spend on anything you want. But they were discontinued in 1970, and once something is discontinued it usually becomes collectible. US Notes were not backed by gold or silver, but they were redeemable for all debts public and private and were supported by the full faith and credit of the US Government. They were issued by the US Treasury Department.
Federal Reserve Notes, the ones we see all the time today, are issued by a PRIVATE corporation...which just so happens to be UNCONSTITUTIONAL, but who reads the Constitution anymore

Not the politicians, that's for sure. (Sorry, got off on a political rant. LOL)
These bills (currency) are backed by the faith people put in our country. This money has no real backup or support. It is not supported by gold nor silver nor the US Government in actuality. They are issued by a private bank that is contracted by the government, hence the name United States Federal RESERVE Bank. It's not really a part of our government but it is treated as if it is. If somebody gave me a fistful of Federal Reserve Notes (our current form of currency) I certainly wouldn't turn them down, but they don't have anything backing them up, unlike in the old days when our currency had REAL value. Of course, since the laws changed, Silver Certificates and Gold Certificates are no longer redeemable for silver and gold, they became virtually the same as Federal Reserve Notes. Except to collectors. Remember, if something is discontinued, it often becomes collectible.
It's not easy to explain these things completely because it is very complicated in many ways. Suffice to say that your US Note is no longer being made because the US Government itself no longer prints money for itself. If you were to read the entire Wikipedia description and history of the US Note you would see what I've been trying to explain here, only Wikipedia perhaps does so in a clearer way. You might want to give it a look-see and read the entire encyclopedic entry there.