Stacking is the term that describes taking several different insurance policies and adding them together for greater benefit amounts. For example, you have three cars insured on three different insurance policies. You have $100,000 of Underinsured Motorist Coverage on each of these cars. If you are in a car accident and your damages total over $300,000, you may be able to collect on all three of these policies.
For more information on stacking, check out this article, "What Is Stacking?"