"Sailing to Byzantium" is a poem of old age. The elderly speaker feels his powers waning, his life force draining away, and so yearns to travel to a distant land for spiritual refreshment. This land is Byzantium. But it's not the real Byzantium that the aging speaker seeks; it's a city of the imagination; an aesthetic creation providing welcome repose for a tired, world-weary soul.
Byzantium as imagined by the speaker contrasts sharply with his everyday world, the world of the here and now. This world is "no country for old men"; a place where the elderly are regarded as little better than "a tattered coat upon a stick." The tattered coat is "mere mortal dress," a symbol of the hold that the world of time and sense has upon us, and which must be broken if the aging speaker is to regain some of his lost energy. And so he embarks upon his journey.
Once the speaker arrives in Byzantium, he exhorts wise men to teach him how to be happy in his soul again:
O sages standing in God’s holy fire As in the gold mosaic of a wall, Come from the holy fire, perne in a gyre, And be the singing-masters of my soul. Consume my heart away; sick with desire And fastened to a dying animal It knows not what it is; and gather me Into the artifice of eternity.
In this stanza, we see an important contrast running throughout the poem: that between the temporal and the eternal. The former relates to that which is born, lives, and will one day die, such as the body. The eternal, of its very nature, will never die. Feeling old and useless in his normal, mundane existence, the speaker seeks out the eternal.
To that end, he wishes to leave his body behind, his soul ascending to an eternal realm. The poem as a whole concerns the renunciation of the world for something higher, more spiritually satisfying. In the final stanza, Yeats symbolizes this aestheticized realm of eternity by a mechanical golden bird, the kind of which was thought by the poet to sing to the Byzantine emperors. Once the speaker's soul has attained this blessed state, then he will feel that his spiritual quest will at long last have ended.