Whole life insurance gives you lifetime coverage at a premium rate that does not increase with your age after you buy. In the early years of the policy, when you're a low risk, you'll pay more in annual premiums than it costs to insure you.
As you become a higher risk at an older age, the level premium eventually becomes less than the amount it takes to insure you. Level premium payments build a reserve in your policy that is used to insure you as you age. Insurance companies call this reserve the "cash value."
Click here for more about the different types of whole life insurance and the different ways to pay the premiums.