And there's a barrel that I didn't fill Beside it, (line 3-4)
This guy is so lazy. Just kidding. But he seems to feel the slightest twinge of guilt about not filling that last barrel. He feels the need to justify his decision to give up for the day.
It melted, and I let it fall and break. (line 13)
The ice is like "glass," but even more fragile, because it can melt as well as break. Fruits, and apples in particular, are associated with the Fall of Adam and Eve after eating the forbidden fruit, so images of falling should immediately catch your attention in a poem called "After Apple-Picking." But this particular image is enigmatic and dream-like.
My instep arch not only keeps the ache, It keeps the pressure of a ladder-round. (lines 21-22)
He feels sore and tired after a long day of work. In the Book of Genesis, after Adam and Eve were expelled from the garden, God further punishes Adam with a curse: the necessity of working for his food by the sweat of his brow. (Eve's curse, by the way, was having extreme pain with childbirth.) In addition to possibly making reference to "Adam's curse," the piece about the ladder is noteworthy for how realistic and detailed it is.
No matter if not bruised or spiked with stubble, Went surely to the cider-apple heap As of no worth. (lines 34-36)
OK, we'll just lay out our cards on this one: the fallen apples seem to us like fallen people, or sinners who have lost their dignity along with their grace. Frost seems to feel sympathy for the fallen apples and, by extension, fallen mankind.
One can see what will trouble This sleep of mine, whatever sleep it is. (lines 37-38)
The speaker will continue to be haunted by similar thoughts all night long, except he won't exactly realize it, being asleep. We think the word "trouble" is a fitting word when you consider all the subtle references to sin and the Fall of Man.