"All risks," like the name suggests, is a type of insurance contract that covers all kinds of risks unless categorically excluded from the contract.
In contrast to a named perils contract, all-risks insurance does not name the risks covered, but instead, it names the risks that are not covered. So, anything that is not named in the insurance is covered automatically.
Physical loss or damage to property triggers the coverage of an "all risks" insurance. An insured must prove that physical damage or loss has happened before the burden of proof shifts towards the insurer, who then has to prove that an exclusion applies to the case.