Normal Yield Curve

Check this out for a jolly-good story to help you visualize the general mathematical concept.

In finance, the normal curve is a reflection of how the price of borrowing money works over time. That is, money due and payable sooner is usually cheaper because risks are perceived as less. Over time, there's more uncertainty; to account for that, those who lend money will charge higher prices to rent that money.

It's normal. The curve shapes from low to high in a twisted smiley kind of way.