What are the greeks "wooden walls"

The Persian army advanced through Attica and entered Athens. The Delphie oracle had said that Athens would he kept safe by her "wooden walls" and a few diehards, refusing to take part in the evacuation, remained behind a wooden barricade on the Acropolis. They learned too late that they had misinterpreted the oracle. Their resistance was overcome and Xerxes burned Athens. Themistocles and his sailors could now be sure that their ships were the only "wooden walls" which stood between the Athenians and annihilation. Unfortunately, although Athens had supplied by far the largest number of ships (180) to the Greek fleet, the admiral-n-chief was a Spartan. Athens had not yet the prestige to persuade other states to serve under her. At sea, as on land, a Spartan had to be in charge.