Our speaker cuts to the chase in this first line. Something is amiss in the world of walls.
"Something" is a wishy-washy word, and just about anything under the sun can qualify as "something." Even a person.
By using "something" instead of "someone," our speaker suggests that humans are not the only wall-destroying culprits around; there are things out there as well.
We begin to channel Sherlock Holmes as we strap on our detective hats. Folks, we are smack dab in the middle of a thrilling, blood-curdling mystery – about walls.
That sends the frozen-ground-swell under it, And spills the upper boulders in the sun,
So, this "something" wields magical powers, apparently. Whoever or whatever it is, it aces Physics, because it knows that water particles swell when frozen, and shrink when warm.
Our speaker hypothesizes that the Something asks nature to cool down the earth below the wall and warm up the boulders in the wall, thus wrecking havoc upon the wall itself. The boulders start to crumble from all of the natural action.
And makes gaps even two can pass abreast.
As a result of said natural action, the wall has gaps big enough for two people to pass through comfortably. Holding hands. That’s a big gap.
And, if two people can pass through the wall easily, just IMAGINE what else can pass through. A golf cart. A baby elephant. A rickshaw. The list goes on and on.