The original question had to be edited. When the speaker, presumably Wordsworth, sees the woman working in the field, he is caught by the image because it is so reflective of the natural world. The "moment" is one in which this woman is bound to the field, both an extension of the other. In a world where so much in way of corruption and impurity is present, this image is the countervailing force.
At the same time, the speaker becomes transfixed with the song that the woman is singing. The speaker is immediately taken into a reflection that moves between the transcendent and beyond the location. The fact that the speaker does not know what the words of the song means enables thought to go to the "Arabian sands" or "the farthest Hebrides." The song is one that invokes the cuckoo and the nightingale. In this immediate sense of reflection, the song enables inspiration to envision what can be out of what is. All this transpires after the speaker saw the solitary reaper and absorbed her song.