She pays 20$ premium per month to the life insurance company. The period of life insurance is for 20 years. So if John dies within 20 years, Sarah will get 4800 dollars. However if John doesn't die within 20 years Sarah will get some money after 20 years which will be much less compared to 4800 dollars.

However if she buys a term life insurance of 4800 dollars for 20 years, she may have to pay premium of less than 20 $, say 10 $ a month. If John dies within 20 years Sarah will get the death benefit of 4800 dollars, however if John doesn't die within 20 years, Sarah will get no cash value at the end of 20 years. However since she has paid only 2400 $ as premiums, her 2400$ are saved as compared to the permanent life insurance policy which she can invest and make profit. In the US market the 2400$ if invested wisely would have yielded much more than 4800$ to Sarah in 20 years.

The idea behind term life insurance is to buy a life insurance policy for a period usually one year. The premium (the amount you pay to the life insurance company) is much less compared to a permanent life insurance premium. The insurance can be renewed after the expiry of the life insurance term, but the premium keeps increasing as the insured ages. The higher the age of the insured, the higher is the premium.

Term life insurance is the cheapest life insurance available on coverage to premium dollar basis. The death benefit is non-taxable in the United States and the premium is also deductible from the income to save income tax.