Hull insurance is optional cover for damage to or loss of your own drone: a crash, a flyaway, theft or water damage. It is exactly the thing the mandatory third-party liability policy never pays for — that policy only covers harm you cause to others.
The typical price is 5–10% of the drone's replacement value per year, with a per-claim deductible. One thing to watch: the camera or other payload usually needs its own separate cover — it is not automatically included in the drone's policy.