The reason money works when you pay for things is because people trust in its value. Historically, you could exchange banknotes issued by us for gold. But the link between notes and gold was broken a long time ago, so nowadays it makes more sense to think of money as a kind of IOU (I owe you).
To understand how money works, a good place to start is to think of a world without money.
In such a world, people could barter with one another: a fisherman could trade some of his catch for fruit grown by a farmer, say. Of course, this wouldn’t always be very convenient. For one thing, you wouldn’t always want to exchange goods at the same time as the person you’re trading with.
Using IOU notes could help with this. In the summer, when the farmer gives the fisherman berries, the fisherman could write the farmer an IOU. Later in the year, the fisherman could fulfil his promise by delivering some fish to the farmer.
While this could work on a small scale, in modern society people buy a huge range of different goods and services. If everyone relied on IOUs, it would soon become a very complex system. Because of the chains of IOUs involved, you might end up having to trust people you had never met before. Plus you’d need to trust that all the IOU notes you were receiving were genuine.