The speaker looks at what is underfoot and finds it holds more than the usual ideas of earth or sorrow. The title plays on "understanding": to stand over something is also a way of grasping it. "Ground" and "grief" sit side by side, as if the speaker expected to find only solid earth or some heavy feeling, then discovers something stranger and more physical.
The list that follows is odd and a little unsettling. "The damned" suggests something cursed or lost, while "a sea of feathers" suggests a vast scatter of birds' remains, or at least a huge soft mass. "Plain blood" is blunt and unadorned. The poem ends on something tiny and exact: the legs and body of a tick. The shift from sea to insect makes the ending feel sharp and almost clinical.
The mood is quiet, sombre and observant. The speaker doesn't wail or explain, just notes what is there. That restraint lets grief show through the details themselves.