Commercial Umbrella Insurance is an additional layer of protection for liability claims that exceed your other liability limits. Meaning if your company is found liable for a large financial sum, first your other liability coverage (such as a General Liability policy) would kick in to cover the amount, and then your umbrella policy would respond to cover the remaining amount up to your policy limits.
For example, if your company is sued and found liable for $1.5 million, and your general liability limit is $1 million, without additional coverage you would be responsible for the remaining $0.5 million dollars. If you have a commercial umbrella policy, it would kick in to cover the additional $0.5 million (subject to the policy coverage limits).
Due to the nature of how Commercial Umbrella insurance policy pays out, it cannot be purchased as a standalone policy - it must be purchased in conjunction with other liability coverage. However, it typically offers a significant liability coverage increase at a minimal cost.