An agreed upon value system of exchange. I have an income of X value, I can buy things with that income because others have things that I value and agree to purchase, using money. I don’t have to go perform labor for farmer John, for eggs, milk, etc., or for the bakery, the butcher, the utilities, for whatever, because we all agree on money as a convenient method of exchange.
Greed is what makes it corruptible. Banks aren’t even the problem. Fake money is. The Fed, here in the US, has the ability to create floating decimal-places of money, based upon a percentage of debt. That fake money circulates digitally, creating more debt. That artificial debt is the primary cause of inflation, devaluing the currency, since it doesn’t represent real property, or labor.