Perils refer to causes of loss. A named-peril is explicitly mentioned in your insurance contract, such as fire lightning, vandalism, falling objects, etc. Your insurer does not cover any losses due to perils not mentioned in the policy.
On the other hand, in case of open peril, your insurance covers all reasons for losses except for those explicitly excluded. Usually, standard exclusions are flood, seepage, earthquakes, fungus damage, etc.