Perils are the causes of loss or damage to property. Some examples of perils are fire, burglary, and water damage caused by a burst pipe. An "all risk" policy is a policy that covers all perils other than the ones listed in the "exclusions" or "limitations" portion of the contract. (The other type of policy, "named perils," only covers those perils specifically listed.)
Although an all risk policy may sound comprehensive, you should still read the list of exclusions carefully to make sure you're getting the coverage you expect.