Burns's narrator addresses the mouse in this poem whose nest has been upended by a plough in November, just as winter is coming. He apologizes to the mouse, saying 

I’m truly sorry Man’s dominion 

Has broken Nature’s social union, 

An’ justifies that ill opinion, 

          Which makes thee startle 

Despite its whimsical title, this poem treats a very serious human problem: how to reconcile planning with Fortune.  We, as humans, think that disasters can be prevented by careful planning, by looking forward.  But when our carefully laid plans are ruined by what looks like fate or a malevolent god, we are bewildered and terrified.  So, in this poem, as Burns looks down at the petrified mouse, he is thinking of how Mankind feels when God’s will, in the form of storms or other natural disasters, ruins our plans.  It is a terrifying moment, not just because we have to make new plans, but because we are reminded that we are subjected to larger forces than our will.  By equating the mouse with human beings, Burns reminds us of our own frailty and our helplessness if “God” has other plans.  Our “best laid schemes” are nothing compared to the forces above us.