An umbrella policy adds a layer of limits above the sub's primary coverages. If a large claim blows through the $1M general liability limit, the umbrella, often $1M to $5M, picks up the excess, and usually follows the same additional insured terms as the underlying policy.
On bigger or higher-risk projects, GCs raise total required limits by requiring an umbrella. Verifying which underlying policies the umbrella sits over is important, since not every umbrella covers auto or employers' liability.