"This type of insurance supplements your existing General Liability and Business Property and Commercial Auto policies to cover certain catastrophic liability claims. This means that if losses exceed the limits of your general liability coverage under your General Liability and Business Property or Commercial Auto policies, an Umbrella policy may cover the remainder, up to a specified limit. Although the coverage provided under Umbrella policies vary, these policies generally cover a variety of business liability risks." Hope it helps :)
I do agree that Amanda's post that Umbrella insurance covers the risk factor involved in business liability and factors related to risk upto an extent.