In the first few verses, the speaker (likely Coleridge himself writing to his then-fiancee, Sara) remarks on the beautiful twilit, pastoral scene around them. He draws attention to the jasmine and myrtle flowers—symbols of innocence and love, respectively. He is enchanted by the scents and the silence of the scene.
In the second stanza, the speaker comments upon the sounds of an Eolian harp, which is a sort of lute without a handle that one places outside or at a windowsill; it plays a kind of music when the breeze blows across the strings (Eolus was the Greek king who controlled the winds in mythology, and the harp is named for him). The speaker hears the music of the harp, and it seems to spark his imagination. He begins to imagine elves and fairies, the harp's relation to the human soul, and the way it compels him to love a world in which nature can produce such beautiful and enchanting sounds.
In the third stanza, the speaker feels that his brain is "indolent and passive" like the harp itself and that "idle flitting phantasies" traverse it, like the wind across the harp. His thoughts seem random, like the "random gales" that produce the sound. In the fourth stanza, the speaker imagines that everything and everyone in nature is really just a harp, and God is the wind that animates them.
In the fifth and final stanza, Sara apparently reproaches the speaker for having such ideas. He ultimately believes that he is incredibly lucky to have been "healed" by God, as he is a "sinful and most miserable man" who is now so fortunate as to have "Peace" and this beautiful spot with his "heart-honored Maid."