Money

Some people will tell you that money is the root of all evil and that it'll only cause mo' problems. But Adam Smith is convinced that money is one of the greatest inventions that humanity has ever come up with.

Before money, people could just trade stuff for other stuff. But say you were a beermaker and the food seller didn't want any beer. Then you wouldn't get any food. As Smith puts it:

[the beermaker and breadmaker] have nothing to offer in exchange, except the different productions of their respective trades, and the butcher is already provided with all the bread and beer which he had immediate occasion for. (1.4.2)

In other words, you're out of luck if you produce something that the other person doesn't want. That's why you need some unit of value (like money) that you can use to buy stuff from people who don't necessarily want your stuff.

The funny thing about money is that it can be almost anything as long as people agree on its value. As Smith notes:

Salt is said to be the common instrument of commerce and exchanges in Abyssinia; a species of shells in some parts of the coast of India; dried cod at Newfoundland; tobacco in Virginia. (1.4.3)

No matter what you use, the important thing is that you and everyone else in your society agrees that money symbolizes value that can be used to buy stuff. The only reason a person will sell you something for money is because they can then use that money to buy stuff they want.

So at the end of the day, money is more a symbol than a physical thing. Its value is totally made-up and symbolic