Not dollars specifically, but rather their market value.  You contend that $1 holds the same value as a McDonald's burger.  However, I'm asserting that this equivalence holds only under the condition that the US government and central bank exercise responsible monetary policy.  If, for instance, they opt to print $10 trillion out of thin air, the value of the dollar diminishes.  This principle is grounded in basic economics—supply and demand—and applies not only to assets and products but also to currency itself. 

I'm intrigued as to why you find this concept challenging to understand.