Farming

The solitary reaper is a… well, reaper, which means she's a farmer. The speaker always tells us what farm work the woman is doing: reaping, bending over her sickle, cutting and binding the shafts of grain. Clearly, it is just as important to the speaker that this woman is a farmer as it is that she is a singer. Farm labor, singing—these things are associated in the speaker's mind. He may even be implying that one cannot exist without the other.

• Line 3: The woman is both reaping (cutting wheat) and singing. We sort of feel that reaping and singing go together, as if the woman's farm work somehow makes her song possible. 
• Lines 5-6: We are reminded again that the woman is farming by herself. This time she's collecting the grains together. Interestingly, "grain" rhymes with "strain" (song), which suggests again the close relationship between farm labor and the ability to sing. 
• Lines 27-28: The woman sings even as she bends over her sickle. The singing symbolizes poetry, while the sickle symbolizes farm labor, so it sounds like the two go together.