There are two sources of money: labor, natural resources, and efficiency. All wealth comes ultimately from one of these three things. Say you borrow money to buy piece of land. How are you going to pay for it? Well, let's say there are 50 trees on the property. You can chop one of them down, plane it into boards, and sell it as lumber.

Since trees grow back in about 50 years, you can chop down a tree every year and sell the lumber. Thus, you have generated a perpetual source of income.

Now let's say that after a few years, you find ways to make trees grow twice as fast. Now you can chop down two trees a year instead of one, and you can double your income. But that's only the beginning! You can go to all the foresters in your neighborhood and teach them your technique in exchange for a cut of their new profits. They'll pay you a fee because their profit will go up dramatically even after they pay you. You're making money because you're showing them how to use their natural resources more efficiently.

A more low-level answer is that money comes from the energy and intellect. Everything can be broken down to the amount of energy it takes to create it--this represents, in a very basic way, its cost. But the energy cost of a process isn't constant; we can invent items and ideas that reduce the amount of energy it takes to accomplish a given task. Thus, intellect and energy together create wealth.