Stanzas are numbered slightly differently in different editions of the poem, but I am going to assume you are talking about the lines,
"With sloping masts and dipping prow, (44)
As who pursued with yell and blow,
Still treads the shadow of his foe
And forward bends his head,
The ship drove fast, loud roared the blast,
And southward aye we fled.
And now there came both mist and snow, (50)
And it grew wondrous cold:
And ice, mast high, came floating by,
As green as emerald".
The passage is describing the ship as the winds blow it swiftly to the south. The metaphor in lines 44 to 47 compare the ship to a fugitive running desperately before his "foe", "bending his head" forward for maximum speed as he surges ahead with the sound of his yelling, striking enemy close on his heels. As he flees to the south, in lines 50-53 the landscape changes, and the area becomes frigid, "wondrous cold", with "mist and snow" and "ice, mast high". The ice is described in a simile as being "green as emerald", and the lines indicate that the ship has journeyed all the way to the South Pole, from its starting point north of the equator.