The speaker is plowing a field and accidentally turns up a mouse's nest. The mouse is shivering and terrified. The man stops his work to try to comfort the mouse.
He tells her to relax. He didn't mean to break into her nest. But then the speaker starts thinking more about it—the mouse is, after all, pretty justified in being freaked out. Mice should be scared of humans. We set traps for them, we set cats after them, and we plow up their winter nests. The speaker apologizes on behalf of all humankind. He says that the mouse might steal little bits of food from human farms, but who cares? That one little mouse doesn't eat much. And now her little winter house is all in a ruin. He imagines the mouse planning ahead carefully for the winter—she worked so hard to make her nice little nest, and then, BOOM. The plow goes right over it.
But hey, says the speaker—that's life. Whether you're a mouse or a man, your plans—however well-laid—often get messed up. And after all, the mouse has it easy, compared to a human. Mice live in the present moment, while humans look to the past with the regret and to the future with fear. Lucky mouse.