The speaker is about to snort a line of powder from a mirror-tray when a spider lowers herself on a thread right in front of his face. Instead of carrying on, he stops and watches. The poem lingers on small details: how she lets out silk with her back legs, steadies herself with the next pair, and feels around with her front legs for somewhere to attach. He gently steers her away from the tray, and when she climbs back toward his finger, he nudges the thread until she cuts it and falls.
What the speaker feels is a mix of tenderness and distraction. He speaks to her kindly ("Not here," "Sorry, sister"), and the Rapunzel comparison makes her seem almost magical. The mood is quiet and wry. The spider's "lines" of silk sit beside the lines of powder, and she lands safely among the newspaper headlines, a small, calm moment in a messy room. The poem doesn't say whether he goes through with it.