Before the Sun is written from the perspective of a child as the day begins. At age fourteen,the boy is able to explain his environment in terms of his tasks or chores and the fact that, at fourteen "big logs / are what you want."
The boy describes the smell of the wood to enable the reader to participate in the wonder of the morning and the task at hand as the "sweet nose-cleansing odour" (odor) enables the reader to imagine a menthol smell, one which actually clears the nose - you can imagine the boy taking a deep breath. To be sure that the reader understands, the boy clarifies how different the smell is from "sawdust" which irritates the nose and would inevitably cause him to sneeze. These literal words are used to give a physical image - a visual picture - to such a simple exercise as chopping wood and the pleasure that you can get from it if you appreciate the surroundings.
This boy relishes the morning and happily prepares the wood as he will be able to cook his "two cobs of maize." The relating of the physical aspect of the morning gives the poem a hopeful tone and the reader is most certainly expecting this to be a good day!