So once upon a time, money was valued by the gold stores a country had. Theoretically every dollar could be exchanged for it's percentage of gold being held by the issuer of the money.
So if the USA had only ever issues one dollar, it would have been worth 100% of their gold. 10$, 10%, $10000000, that fraction of a %, etc.,
This is why just printing more money or changing the Number on a Currency by a decimal (1-> 10, or printing 100 more bills) doesn't change anything- because no matter what, the value of a dollar was based on what tha country was worth in gold- so the fraction of gold it's worth just dropped.
Some countries used other precious metals (silver), but gold was good because it had a steady value- over time gold tends to slowly increase, and it is not typically subject to major drops like silver.
This is not the case anymore.
Today, a currency is valued based on a number of factors- such as the strength of an economy, or the value of natural resources.
In really, really short, the USD is dependan on the promise of the US government to exchange it for a set value of goods- both on an inter-governmental level (between countries) and on a local level (within the USA). As long as there is the perception that the USD is worth something. When the percieved value is higher, it is traded against other currencies for more. When the value is lower, it is traded at a lower exchange.
And that isn't much different then Canada- although I simplified above by saying our dollar is tied to oil/trade, it is actually tied to the stability of those things.
When Canada can promise the world that our currency is stable, our exports are booming and our trade numbers are high, they want our money as a stable platform to do business. This in turn drives its value up. Oil and Trade are just two really good indicators of stability for us.
Like Canada, the USA has a currency based on the strength of its nation, the ability for it to trade, and the garuntee that it wont be taken over tomorrow, and have the USD replaced by an entirely new $, making the old one worthless.
All that together make up only a franction of it's actual value, but it's about as simple as I can get ;/
Hope it helped
Cheers