Money ultimately is trust and perception. We trust that the United States and the vendors here will exchange the paper or digital money we give them for something we desire. Once this trust is in place and we have a system that functions on a particular monetary system with defined characteristics (total money in circulation, methods of exchange, denominations of physical money, means of accounting, etc.) we then assign values to physical things on the basis of that system and our perceptions of those things.
Money has no intrinsic value. You cannot eat it, build a worthwhile shelter out of it, etc. It doesn't meet your physical needs. Thus, without trust people seek desperately to trade their currency for physical goods. This makes the demand for money plummet, while the demand for physical goods simultaneously skyrockets. The purchasing power of that money falls and you get hyperinflation. Examples of this can be seen in the Weimar Republic after World War 1 and Kenya, that both experienced this situation when trust in those currencies fell and the central banks overprinted money.
Another interesting example can be found in fame. Take a sports shoe for example. That shoe may be worth some amount purchased from the retailer. However, say that shoe is signed by an incredibly famous athlete. With a very small value of ink and seconds of that athlete's time, the shoe's value will skyrocket without any addition of utility. The only thing that has changed is perception.
The last example I will point to is the US debt. Currently, the United States claims approximately $20 trillion of debt. The physical currency does not exist. That money could never exist. 20 trillion is an incomprehensibly large number. Never in my life have I ever encountered something, and decided, "Hmm, that seems like around 20 trillion." It is not a physically realizable quantity. In other words, its perception, imagination if you will.
Ultimately, money is a system of perceived value that only functions properly while all parties involved with the currency trust that it will continue to be a means of exchange for physical goods that meet their needs and wants. Any currency that is not trusted will fail.