Decreasing term life insurance offers a death benefit that shrinks over time, and a premium that remains the same for the duration of the policy. Decreasing term policies are usually slightly cheaper than regular term policies, but because they shrink in value over time and because regular term policies are already low-cost, it usually makes better sense to go with a regular term policy.
This type of policy is sometimes called "mortgage insurance" because it is marketed as a way to cover the cost of debt that decreases over time, e.g. a mortgage. However, this is not the same thing as the private mortgage insurance you may be required to buy as part of a home loan.