The speaker of the poem is giving us a very positive estimation of the solitary reaper's beautiful, beguiling song. He compares her song to that of the nightingale, a bird especially renowned for the beauty of its song. One might expect that, though her song is undoubtedly beautiful, the solitary reaper's delightful melody cannot compete with that of the nightingale. Yet the speaker immediately challenges our expectations by drawing a comparison that actually flatters the solitary reaper:
No Nightingale did ever chaunt
More welcome notes to weary bands
Of travellers in some shady haunt,
Among Arabian sands:
In other words, the solitary reaper's song is more beautiful to the speaker than that of the nightingale to a group of weary travelers trudging their weary way through the Arabian desert. If such a group of travelers heard the sound of a nightingale they wouldn't just be delighted to hear such a sound; they would also be relieved to know that they would soon be in the bosom of civilization once more, and wouldn't have far to go before finally putting the burning sands of the desert behind them.
With pardonable exaggeration, the speaker believes that the notes of the solitary reaper's song are even more welcome to him than the sweet melody of the nightingale would be to travelers in the Arabian deserts.