It's silly wa's the win's are strewin! An' naething, now, to big a new ane, (20-21)
The mouse's nest seems to represent the culmination of a lot of hard work and carefully-made plans, and now the walls ("wa's") of the house are being strewn around by the cold winds ("win's"), and the mouse is left without any raw materials to build ("big" is Scots for "build") a new one. Sure, the mouse could continue to hope, and dream, and plan, but without the physical means to make those plans a reality, what's the point? The poor little guy is out in the cold.
Thou saw the fields laid bare an' waste, An' weary winter comin fast, An' cozie here, beneath the blast, Thou thought to dwell— (25-28)
The speaker imagines the mouse observing the weather and the coming winter, and carefully making his little mousie plans for the future. Do mice really hope and plan like this, or is it all instinct? It's hard to say, but it's a good mental image, and anthropomorphizing the mouse (talking about her like she's a human) helps to make the analogy between the mouse and all of humankind a lot clearer.
That wee bit heap o' leaves an' stibble, Has cost thee monie a weary nibble! (31-32)
The speaker drives home the point that the mouse spent a lot of time and hard work building up this nest, and now it's all ruined. Argh.
But, Mousie, thou art no thy lane, In proving foresight may be vain; (37-38)
Don't worry, Mouse (says the speaker), you're not alone. That's what we all want to hear, right? Doesn't misery love company? Well, the speaker assures the mouse that she is not alone in finding that plans can go awry even with all the advanced planning and foresight in the world.
The best-laid schemes o' mice an' men Gang aft agley, An' lea'e us nought but grief an' pain For promis'd joy! (39-42)
Yep, these are probably the most famous lines in the whole poem—and that's saying something. Remember all the work the speaker has done to make a connection between the mouse and himself, and between the mouse and all human beings? This is the pay-off: the speaker says that even the best plans, whether you're a mouse or a man, can get ruined, and leave you in tears when you expected joy. This, says the speaker, is a universal condition for any "fellow-mortal" born on earth. Well, if misery loves company, we're all in good company.
But, Och! I backward cast my e'e On prospects drear! An' forward, tho' I canna see, I guess an' fear! (45-48)
The speaker envies the mouse, in a way, because the mouse only worries about his present condition. The speaker, on the other hand, has to fret about his past and about the uncertainty of the future. If you know that your plans can get screwed up no matter what you do, maybe it is better just to live in the moment, like a mouse.