Vsauce aka Michael Stevens, explains a bit what money are and how much money we can see int he world. Depending on how we look at it, we could have 5 trillion USD (M0) or 75 trillion USD (M3). Money ranges from cash and coins to the liquidities in the bank accounts, which might not even be actual banknotes.
Michael explains more about commodity money, something that has a real value in it and of itself, like water and gold, and about fiat money, something that has value because we believe (and want to believe) it has value. The actual banknotes are worthless, but they have a value based upon mutual agreement that those banknotes really have a value.
In other words, money has value because we say so. The financial aspect of our lives is rules by very strange and interesting facts that gravitate around notions like money, value, trust and beliefs. Vsauce explains all better that many others. Have fun watching the video.