Good will is what makes other people do things for you - providing you a place to live in, giving you a television, giving you a loaf of bread, letting you ride on their airplane, or serving you a meal.
Sometimes we are happy to do things for others, and expect nothing in return, but most of us do expect something in return when we do something for others. When we do something for others, we generate good-will - we have an expectation that they will do things for us in return.
With family and friends most of us don't like to formalize this good-will. We just expect that favors will be returned. Sometimes people decide that insufficient favors are being returned, and people argue with, or disassociate from, friends and family.
The economies of scale in a modern society mean that we will deal with many people who we never even see again, and cannot reasonably expect to return favors. A computer programmer cannot go to a fruit stall and try to swap one line of computer code for an apple. A single line of computer code out of context is useless to anyone. Money allows us to expect the favor to be repaid by others. Money allows the fragmentation of the good will in a large system.
Describing money as formalization of good will may seem strange in the context of bank robberies, stand-over merchants, scam artists and criminal force, but these people are actually stealing good-will. We may think that a small amount of the money we receive has been obtained through force or exploitation, and it has. But a high percentage of the money has been obtained through genuine productivity. If people perceived that 99 percent of the money which circulated in the community were stolen they would not bother to trade with money. They would simply find something else to deal with - probably just old fashioned barter, and the money system would collapse. All systems have parasites.
Significantly too, money is not the little green bits of paper with pictures of popular feminists or the words "In God we Trust" on them. This is just a form of documentation called cash. The contents of bank accounts is also money - even though there is no such cash related to it (banks have cash but not enough to pay all their investors - the money is invested elsewhere in the form of loans).
Anyone who says "there is only so much money in the world" and starts talking about the amount of cash is ignorant. The total amount of money in the world is not just the amount of cash, but includes the amounts in bank accounts and the amount of money lent out based on those accounts. In a sense the same cash can be counted many times. The details can be found in any first-year economics textbook.