To a Mouse
"To a Mouse" narrates such a personal incident that most readers are tempted to identify the speaker as Robert Burns himself—after all, Burns was the son of a farmer in Ayrshire, Scotland. In fact, Gilbert Burns (the poet's brother) wrote that Robert actually stopped and composed the poem while plowing the field (as opposed to later, in retrospect).
But that could, of course, be a total fantasy. It's usually not a good idea to identify the speaker of any poem with the poet him or herself, and this is no exception. Even in poems that are explicitly autobiographical, they only reflect a little sliver of the poet's persona in that one particular moment in time—so it's always safer to refer only to the "speaker" of a particular poem, rather than to use the poem to make general claims about the poet.
So what do we know for sure about this speaker? Well, he's a farmer, of course—he's plowing a field before the onset of winter and accidentally turns a mouse out of her little nest. He's a soft-hearted guy—instead of immediately continuing with his chore, he stops to apologize to the mouse. In the process, he starts to reflect about the plight of mice in general, and realizes that uncertainty about the future and the overthrow of carefully-made plans isn't just a problem for mice—it's the problem for any "mortal" and any "earth-bound" creature, whether a mouse or a human.
Now that's a pretty deep thought for a stereotypical rural farmer in the 1700s. Not only is he able to empathize with the plight of a rodent in his field, he's able to extrapolate from the mouse's situation to comment on the plight of every living thing on earth. Wow.