The speaker is overwhelmed by the beauty of the woman's voice and her "melancholy" song as he passes by the field where she works. He compares her lovely voice to birds, suggesting that her singing is actually more beautiful than theirs:
No Nightingale did ever chaunt
More welcome notes to weary bands
Of travellers in some shady haunt,
Among Arabian sands:
A voice so thrilling ne'er was heard
In spring-time from the Cuckoo-bird,
Breaking the silence of the seas
Among the farthest Hebrides.
He claims that no nightingale's song was ever more welcome to weary travelers in faraway and foreign places than the Scottish girl's is to him. Further, he claims that no cuckoo's voice was ever more welcome to those on the open sea than hers is to him.
Her song has rendered him "motionless and still," as (though he cannot understand her language) it gives him an impression of "natural sorrow, loss, or pain" and "old, unhappy, far-off things." In fact, the sad-sounding mood of the song stays in his heart, affecting him quite deeply, even "Long after it was heard no more."