The poem sets two pictures side by side. In the first, a day is described as a lash, something that stings and keeps on stinging, and it has no laughter in it at all. The speaker doesn't say who is being whipped or why, so the hardness feels general, the kind of day most people have known. Then the poem shifts to a concrete scene: an old quarter horse that simply stops in the middle of the road.
The two parts aren't tied together with an explanation, but the title, "Impasse," nudges us to connect them. The horse seems to stand in for the speaker's own state, worn out and unable to go on. Its age matters too, since it suggests a long stretch of work and not just a bad afternoon.
The mood is weary and quiet. Nothing dramatic happens, and the stillness is the point. There's also a hint of sympathy in the way the horse is seen: stopping may be the most honest response to a day like this.