the dollar started as a receipt for gold thus its value was the same as the value of gold you could get for it.
over time to finance public works and war the government started printing more dollars and eventually had to give less and less gold per dollar in existence (they couldn't print gold to back up the paper).
eventually in 1971 the government had so little gold and so much paper in circulation that it had to stop redeeming paper for gold altogether.
since then more and more paper was printed the the purchasing power is declining ever since.
"Fiat" is not really fiat. It is in places like Zimbabwe, however in the US it is credit money. Every single credit dollar in existence is backed by collateral. The federal reserve asset column consists of mainly US government bonds. What are US government bonds? These are future claims on the sweat equity of 310 million Americans. So every dollar the fed lends into existence is collateralized by the future productivity of American's which will be extracted via taxation. Now ask yourself, does future productivity have "intrinsic value"? Of course it does.
this is not true, the dollar has no collateral backing it, not even government bonds (who are nothing but papers as well anyway), commercial banks are allowed to create as much currency as they want they don't need any government bonds to back that currency.