The winter is coming quickly, and the mouse has been turned out of his nest. Poor little guy might either freeze or starve. As the poem progresses, the speaker reflects that the plight of the mouse is actually a lot like the plight of all human beings—especially the poor. Like the mouse, poor folks have to worry about the coming of the winter. And all people, rich or poor, have to worry about the uncertainty of the future and the possibility that their carefully laid plans will get all screwed up. So, in a sense, the coming winter in this poem could be read as the scary, uncertain future.
Line 21: We get more alliteration here with the repeated N sounds of "naething," "now," and "new." The alliteration adds to the staccato effect of the line, and might draw attention to the word "naething," emphasizing that the mouse really has nothing ("naething") left, now that the plough has destroyed his home just before the winter. The pauses at the commas in the middle of the line around the word "now" are an example of caesura. These commas encircle the word "now"—the present moment—and add to the emphasis on the mouse's present problems and the impending cold of winter.
Line 26: More alliteration. The repeated W sound of "weary winter" actually sounds cold and weary, like the whooshing of the wintry wind, doesn't it?
Line 35: More onomatopoeia here—the word "dribble" is one of those words that sounds like what it means.
Line 36: The speaker uses alliteration with the repeated hard C sound of "cranreuch cauld"—maybe to emphasize the hard coldness of the wintry frost.