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Money changes in value over time.
If I said I could give you $10 today, or $10 next year, you'd rather have the money now, because you can do something with it, instead of just waiting for it.
So we'd say that ten dollars today is worth more than ten dollars next week which is worth more than ten dollars next year. That's the basic idea behind interest.
Time value of money is just a way to figure it out quantitatively (using numbers). If ten dollars next year is worth less than ten dollars today, how much less? If I offered you $9 today, or $10 next year, maybe you'd change your mind.
We'd say that the NPV, the *net present value* of $10 next year is whatever amount you'd be willing to trade it for today. If you'd rather have $9.51 today than $10 next year, but you'd rather have $10 next year than $9.49 today, we'd say that the NPV of $10 next year is $9.50.
This is done to help calculate expenses and investments. If I said I would give you $10 today, $11 next month, $13 next year, $20 next decade, or $100 in a century, which would you rather have? Not an easy question, til you calculate the NPV of each.