As the name implies, it is a type of life insurance coverage that decreases its payout as time passes.
It was used in the past as a type of mortgage life insurance. Basically, as the mortgage was being paid down, the life insurance amount was also being decreased.
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You are much better off simply getting a term life insurance policy to cover your mortage in the event of your or your spouse's death.
For example, if you have a 20 year $500,000 mortgage, simply get a 20 year term life insurance policy for half a million dollars. This way, even if you die 19 years from now, your spouse will still get the full amount of the policy ($500K).