What does stanza one mean?

The poem is (George Gordon) Lord Byron's 1815 recounting of the Old Testament story of King Sennacherib's failed assault on Jerusalem. It was part of a grouping of poems the poet composed and collected as "Hebrew Melodies."
In the first stanza, "The Assyrian" refers to King Sennacherib, and the poet uses a simile in the first line that compares his siege against the city of Jerusalem to a wolf attacking a fold (flock) of sheep. Sheep are traditionally used as a metaphor for Christians. "His cohorts" are the men in the Assyrian army who are resplendent in their colorful garments of war. Byron next uses another simile to describe the soldiers' spears, likening them to the glimmer of stars that reflect on the sea of Galilee each evening when the "blue wave" of night arrives.