Accidental Damage covers unintentional damage to the building or its contents. For example, putting a foot through the ceiling whilst in the attic above, or knocking over the TV.
Accidental Damage is an optional cover which provides protection for events not covered by the normal perils of fire, flood, theft, etc. You can have both accidental damage insurance for buildings and accidental damage cover for contents. When taking out home insurance always check what exactly is covered under accidental damage. Make sure you check the terms and conditions of your policy because if you have accidental damage cover for your building it doesn’t necessarily mean you have accidental damage cover for your contents.