The speaker imagines being crushed by the Universe the same way they crushed a spider on their dining table. The spider was big, black and beautiful, and the speaker admits they didn't need to kill it. It just startled them among their papers. The image of its legs curled up afterward lingers, and it becomes a mirror: the speaker, safe in a "nest," is found and flattened by something far larger and just as careless.
The mood starts out guilty and shaken, then shifts. The Universe is pictured as a place of "speckled wastes" and "void vasts," cold and indifferent. Yet the ending turns strange. Being squeezed "awake" forces the speaker to put the Universe's emptiness into words, and the pain they feel ("traumic anguish") somehow makes that emptiness "thrive." So the poem moves from guilt to helplessness to a bleak sort of purpose: suffering becomes the thing that gives voice to a silent cosmos.