Sunset and evening star, And one clear call for me! (1-2)
These first lines tell us that the speaker has reached the "sunset" and "evening" of his life. He's old, the end is nigh, etc. etc. What's really interesting is how he reacts to that fact—with serene resolve, rather than frantic freaking out.
Twilight and evening bell, And after that the dark! (9-10)
Twilight can refer to the time just after sunset and the time just before dawn. Sure, the speaker is old—in the twilight of his life. But the fact that twilight can also be associated with dawn suggests that old age might also herald the beginning of a new day, a second youth.
And may there be no sadness of farewell, When I embark; (11-12)
The speaker shows the wisdom of old age in these lines; he doesn't want there to be any "sadness" when he crosses the bar. Perhaps this is because he has accepted death, and realizes that it isn't the end. Or maybe he's just trying to save his loved one's a little pain. Either way, good move, dude.