The title plays on the phrase "a bone to pick," meaning a grievance you want to argue about. The poem takes that idea literally and stretches it across a landscape. The hill has a "body," and the people travelling over it are picking at it, or at each other, the way scavengers might gather around a carcass.
The middle lines are plain: "we quarrel / on a journey." Nothing explains the argument. That bareness makes it feel small and familiar, the kind of bickering that happens when people are tired and stuck together on the road.
The last stanza widens the view. The speaker asks us to "think how people are / at a feast of stone." A feast should be generous, but stone can't be eaten, so the image is faintly absurd and bleak. The speaker seems wry and a little weary, watching people squabble over something hard and barren. The mood is quiet, dry, and gently mocking, with the vast, indifferent hill dwarfing the human fuss.