This poem opens with Wordsworth describing how a solitary girl from the Highlands of Scotland is singing a beautiful song while she is reaping grain. Although she is just a common girl out in the country, he goes on in the second stanza to compare her "melancholy stain" to things that have inspired many poets.  As the enotes entry for this poem states:

He associates the sweetness of the reaper’s song with the beautiful cries of the nightingale and the cuckoo, both familiar images of transcendence in Romantic poetry.

In addition, I would note that there are all sorts of oppositions and expansions in this stanza.  For example, the Arabian sands (desert) are opposed to the seas of the Hebrides (way in the north of Scotland).  This is also north-south opposition.  The travellers listening to the nightingale are weary and in the shade, while, presumably, other travellers in the rough and cold Hebrides are thrilled by the song of the cuckoo.  Both the nightingale and cuckoo's songs are heard in places often devoid of life (desert and ocean).  So, too, this solitary maid is in the rugged and lightly populated mountains of Scotland.  Finally, in English poetry, the nightingale is traditionally associated with immortality (e.g., Milton, Keats).  The cuckoo is associated with spring, which in turn leads to ideas of newness and resurrection.  This idea is expressed in both this poem and more extensively in Wordsworth's "To the Cuckoo."