The sunflower symbolizes somebody who wishes to achieve serenity at its peak once ending the path they're following.
The sunflower is known to be a flower that always faces the sun, avoiding the dimly lit scenery from behind, which explains the lines "Who countest the steps of the sun; Seeking after that sweet golden clime".
In this case, the search for the sunlight represents going on a journey and looking forward to its end, which explains the line "Where the traveller's journey is done".
Finally, the lines "Where the Youth pined away with desire, And the pale virgin shrouded in snow" describe the scenery of wherever the sunlight resides, where youth prevails (pinned away with desire) and the environment is untouched by foul things (like a virgin shrouded in snow).
