When someone in one country wants to buy a good or service from a different country they cannot pay in their local currency because the foreigner isn't going to want their paycheck in that currency. So they have to exchange one currency for another, and there is a pool of this currency held by those who import and export to facilitate such exchanges.
When the dollar was dropping like a stone it really screwed the EU economy because of this shit.
The american company that baught EU goods had the bill in dollars. so at day 1 when they get the bill they owe say 1000 $ witch was say 750 euro at day 30 or 60 when they had to pay the bill they still owed 1000 dollars witch was say 500 EUR.
Well on the bright side mobsters are very interested in the EURo cause we have a frigging 500 EUR bill currently worth 684.514 US dollars right now kinda helps with the shipments of cash if you dont have to carry tones 100$ bills when you can only carry 1/6 of the volume.