additional coverage...usually for those who have a lot of assets, income, etc. it goes above and beyond their auto and home insurance coverages. this way, instead of getting a 1mil/1mil coverage on auto and 1mil on home, then can get 500/500 on auto and 500 on home and then just get a 1mil coverage on their umbrella for a couple hundred bucks but end up with a 1.5mil coverage as opposed to a 1mil limit on one policy.