Vacant land is land that has no fixed structures on it. No buildings, and not structured permanently attached to/in the ground.
Damage to land is not something that you can normally insure against, so there's really no kind of property damage that is a typical concern. However, vacant land can definitely be a liability concern.
Vacant land is generally covered for liability risks automatically on the owner's home insurance or renter's insurance policy. But you should if you own land that is anything other than literally vacant, you should discuss this with one of our agents.