There are a number of themes apparent in this haunting, romantic poem. First, there is a sense that nature possesses mystical and healing powers, and that being in nature can cause true believers to have visions. Yeats wrote often of fairies, a pervasive presence in Irish mythology. He also seemed to think of some of the human women in his life as having fairy-like qualities.
The "glimmering girl with apple blossom in her hair" is a fairy being, beautiful and desirable, but unobtainable, just as Maud Gonn was, Yeats' unrequited love. The protagonist of the poem wishes to return to the same location, the hazel wood (hazel trees possess magical powers in Irish folklore), to see the girl again. There is a theme of timelessness to his longing, and his hope. The fairy is a being who will always be in this place, and the speaker is confident that when he is "old with wandering" he will see her again.
In this way, the fairy is also a metaphor for the peace and completion of death. The realm of the fairies was comparable to the underworld, of the realm of the dead, and in some locations humans, under certain conditions, could enter the world of the dead and return to the world of the living. The "silver apples of the moon and golden apples of the sun" also reflect themes of timelessness and immortality, a vision of the afterlife in which the beauty of nature is a soothing presence.