。
Many trees were blown down by the storm.
Literal
Storm [by-で] many [attributive-の] trees [subject-が] were-blow-toppled.
吹き倒す is a compound verb: 吹く ('blow') + 倒す ('topple/knock over'). Japanese routinely stacks two verbs this way to express tighter, more specific meanings. The passive 吹き倒された describes the action from the trees' perspective. Note how the intensive 倒す (transitive 'topple') pairs with the passive to convey 'toppled by being blown.'