What does the line from the Old Ironsides poem “ Where knelt the vanquished foe” mean?

The foe, the enemy, that got onto the deck but was cut down to their knees before they died.

IS THIS FROM MONTY PYTHON AND THE HOLY GRAIL

It's a syntactic inversion; it means the same thing as "where the vanquished foe knelt," or "where the defeated enemy was kneeling."

Ay, tear her tattered ensign down! Do they still teach that in school? The line means that during the battles the ship was involved in, the enemy sailors were forced to surrender and kneel on the ship's deck.